Materials Science in Reference to Aquariums

[Sarina]

This will be an ongoing journal of my materials science knowledge related to aquariums. Questions welcome. Please alert me of any errors or new science that I am unaware of, I am always open to learning and updating my knowledge!

My credentials:

~Certification of completion in Composites Materials Science from Lake Washington Technical College
~5+ years of experience in the fields of woodworking and composites fabrication
~63 credits of materials science and chemistry courses at various technical/higher education institutions
~9+ years of experience as a hobbyist fabricator

Glass and silicone

Glass and silicone are unique materials, and are oddly similar in chemical properties in spite of their very different physical properties. Both materials have a molecular structure known as a siloxane bond, made up of silica and oxygen. The most important thing to note about siloxane bonds is that they are very strong. Materials made with these bonds are resistant to chemical degradation, extreme temperatures, and even radiation. 

Because glass is so chemically resistant, it is very difficult to glue together with materials that do not share a similar molecular structure. This is why silicone is our go-to glue for aquariums. Even though Silicone is the most effective glue, it still is not perfect and degrades at a very fast rate in comparison to glass. This is because in order to maintain the flexible physical properties that silicone is known for, it contains organic (carbon atom) compounds that can change certain physical properties about it. The organic compounds found in silicone can alter its shore hardness (how firm the silicone is), its UV resistance, heat resistance, and whether or not it remains a liquid or cures into a solid. It should be noted that silicone that cures into a solid does so thorough a chemical reaction, and once it is cured it is very non-reactive. This type of polymer is called a "thermosetting" polymer. Polymers that can be melted into a reactive/liquid state again are called "thermoplastic" polymers. There are other factors that can be affected as well, however these are the most obvious and useful to us in this hobby. 

Because silicone degrades much faster than glass, is so chemically resistant, and is only in a reactive state when it is first applied, old silicone does not stick to new silicone. This is why when an aquarium needs to be re-sealed, you cannot just scrape the inside bead and put a new one down, you must take apart the entire tank and scrape off every tiny bit of old silicone and rebuild the whole thing with fresh, new silicone. Any old silicone that remains stuck to the glass will create a potential path for water to escape, and so will any material that prevents the silicone from adhering to the glass. It is advised to wear gloves and clean the glass panels thoroughly with alcohol, as even oils from your fingers can disrupt the silicone's adhesion to the glass. 

One of the most common myths I see about aquarium silicone is that chemicals and medications can seep into the silicone and then leech into the aquarium water later on down the road. This is untrue, silicone is incredibly resistant to chemical penetration due to the siloxane bonds. What can happen is silicone separating from the glass in certain places, and things getting stuck in those spots where it can't easily be cleaned. It is also likely that certain chemicals have leeched into the lid and upper frame of the aquarium, and evaporation/condensation washes those chemicals back into the water. Filtration equipment can also be made of more porous plastics, making chemical contamination a higher risk for those items. 

Plastics

• PVC 
  • Thermoplastic.
  • Can be glued using several types of adhesives that include solvents. 
  • Generally safe for aquarium use.
  • Can release chlorine gas if heated, heat bending requires proper ventilation, do not melt.
• ABS
  • Thermoplastic.
  • Can be glued using solvents.
  • Generally safe for aquarium use.
  • Heat bends very easily, but melting can release toxic fumes. 
  • Great 3D printing material for aquariums, but requires proper ventilation. Can be used for structural components as it does not break down as quickly as other 3D printing materials.
• Acrylic
  • Thermoplastic.
  • Can be glued with solvents and UV epoxy. 
  • Two kinds, extruded and cast.
    • Cast acrylic is much stronger because the crystalline structure is allowed to form naturally. Cast acrylic is stronger, visually clearer, and more heat and chemically resistant. This means that extra care needs to be taken when gluing and polishing cast acrylic. 
    • Extruded acrylic is forced through a die, so it is more consistent in thickness, but it is structurally weaker and less visually clear when compared to cast acrylic. 
  • Popular choice for aquarium building due to high clarity and strength, but needs more structural support due to being less rigid than glass.
  • Hygroscopic, meaning it absorbs water over time. This can cause bowing, overall degradation, and weakens the crystalline structure over time. 
  • Does not make a good aquarium lid in spite of fantastic light transmission, due to the tendency to bow from hygroscopic absorption. 
  • Scratches easily, meaning it may need regular surface restoration to maintain clarity over time. Cleaning algae can be difficult 
  • Acrylic is susceptible to something called "crazing" which is a crackled appearance along the surface of the plastic. This happens when the crystalline structures of the plastic are broken down by chemical, heat, and/or UV exposure. Crazing cannot be repaired, so tanks with this type of damage are to be avoided at all costs. Crazing weakens the structural integrity of the plastic and makes it more brittle, a crazed area of acrylic can spontaneously burst under pressure, or crack if impacted the wrong way. 
  • Bonus: Acrylic paints
    • All basic acrylic paints, including aerosols, will be aquarium safe once cured. 
• Polycarbonate/lexan
  • Thermoplastic.
  • Very impact resistant and much higher chemical resistance compared to acrylic. 
  • Chemical resistance makes this plastic difficult to glue with solvent, or any other material. 
  • Not as visually clear as acrylic, but allows light to pass through just as well. 
• PLA
  • Thermoplastic.
  • A popular choice for 3D printing, non toxic and very predictable.
  • Made with plant-derived polymers, so this material breaks down faster when exposed to water. Parts made with PLA should not be structural, but filtration upgrades, decor, backgrounds, etc are fine. 
  • Can be glued with solvents.
• PETG
  • Thermoplastic.
  • Lesser-known 3D printing material.
  • Aquarium safe and does not degrade as quickly as PLA, but is more affected by chemical, temperature, and oxidative stresses than ABS. 
  • Can be clear, makes excellent custom tubing. You can purchase "PC cooling tubing" and heat bend it to create your own intake and outflow for filtration. 
• Polyester resin
  • Thermoplastic.
  • Not recommended for aquarium use unless sealed. Polyester resin is a thermoplastic that cures by quickly evaporating the solvent that melted it with a catalytic additive. This solvent can continue to leech from the plastic for a very long time. Not as UV resistant as other materials.
  • Can be glued with solvents.
• UV cure resin
  • Thermoset.
  • Aquarium safe once cured. Great for gluing applications, but degrades over time with exposure to UV. 
  • Cannot be glued with solvents.
• Two part epoxy resin/putty
  • Thermoset.
  • Comes in a variety of colors, clarities, and hardnesses. 
  • Aquarium safe unless it contains additives are not aquarium safe. Some epoxy putties are impregnated with metallic compounds to increase weight and strength, these should not be used in aquariums. 
  • Some can be applied and cured under water without harming aquatic life. 
  • Cannot be glued with solvents. 
• Polyethylene
  • Thermoplastic.
  • Extremely chemically resistant, so this material cannot be glued with solvents. 
  • Used to create fittings and other various aquarium components that need high strength and high chemical resistance. This type of plastic is used frequently to make power head and filter impeller blades.
  • Can be heat formed, but only with proper ventilation as melting this plastic will create toxic fumes.
  • Can only be heat-welded using special equipment. 
• Styrene/polystyrene
  • Thermoplastic.
  • Brittle, degrades quickly, very cheap.
  • This type of plastic is what foam packing peanuts and foam padding is made of. 
  • Can be clear, but does not have good visual clarity. 
  • Can be glued with solvents. 
  • Produces toxic fumes when melted, not recommended for heat forming. 

Concrete/cement

• Cement is the powdered stuff used to make concrete. These two materials are often conflated, but this is similar to conflating flour and cake. 
• Cement is a mixture of Lime, silica, alumina, magnesia, sulfur trioxide, iron oxide, and calcium sulfide. 
• Cement can be mixed with a number of different aggregates to achieve different textures, strengths, and appearances. 
• The typical concrete mix is made up of roughly 10% cement, 20% air and water, 30% sand, and 40% gravel. This is known as the 10-20-30-40 rule. 
• Concrete will harden water and raise pH, as it contains lime. Coating concrete with a surface treatment will prevent this. Drylok masonry paint is a popular choice, but acrylic paint, pond sealant, and liquid flex seal are also great options. 

 

Foam

• There are two types of foam, open-cell, and closed-cell. Open cell foam is the foam that allows water and air to pass through it. Closed-cell foam is impermeable and is commonly used in gap filling, waterproofing, and insulation. 
• Almost all foam that does not degrade in water is safe for aquariums. Great Stuff foam is commonly used for 3D aquarium backgrounds as it is a cheaper alternative to the Pond and Stone variety, but is nearly the same material. 
• General purpose urethane foams must be used with caution, however most are stable and safe when cured. 
• Latex foams are not recommended, see section on latex below. 

 

Other stone/rock products

• Most stone and rock is safe for aquariums unless there is a high metallic content. Rocks containing lime will raise pH, KH, and GH, but are not toxic. 
• Quartz-based rocks are mostly safe, especially clear crystalline quartz. 
• The vinegar test is a great way to tell if a rock may contain lime. 
  • To perform the vinegar test, drip some vinegar in one spot on a rock you wish to test. If you notice any fizzing, or any sort of reaction, the rock contains calcium carbonate and will affect the water chemistry of the aquarium. 
• Crystals and stones that are known to be toxic to aquatic life include, but are not limited to:
  • Malachite
  • Azurite 
  • Chalcopyrite 
  • Lapis Lazuli
  • Turquoise
  • Chrysocolla
  • Lepidolite
  • Actinolite
  • Amazonite
  • Angelite
  • Garnet
  • Hematite
  • Labradorite
  • Lodestone
  • Pyrite
  • Serpentine
  • Tiger's Eye
  • Unakit
  • Cinnebar
  • Chalcanthite
  • Stibnite
  • Torbernite

 

Latex, urethane, and other rubbers

• Latex
  • Latex is a natural rubber, derived from the rubber tree. It is one of the few rubbers that is 100% biodegradeable.
  • Due to its high biodegradability, latex is not recommended for use in aquariums. The breakdown of this rubber can cause algae outbreaks.
• Urethane
  • Urethane rubbers are typically used in tires, they are high strength, however they tend to oxidize and degrade quite quickly. 
  • Because of how quickly they degrade and how chemically reactive their makeup is, urethane rubbers are not considered aquarium safe. 
• Vinyl rubbers
  • Most vinyls are inert once formed/cured, and are safe for aquariums. Lots of airline tubing is made out of vinyl.
    
     

Wood

• Most non-aromatic hardwoods are aquarium safe when fully dry. Aromatic hardwoods may still be used if they are properly seasoned and all aromatics/resins are oxidized/degraded. This process can take many years.
• To be the most safe, avoid coniferous/aromatic species as these can sometimes take much longer to cure fully and aromatics can be toxic in an enclosed ecosystem. Conifers like pine will leak resins long after being dried, which makes them absolutely unusable in an aquarium.
• Woods that cannot be used in aquariums are those that are too soft and which rot too quickly, causing a decomposition overload that the closed ecosystem of an aquarium. 
• You can "cure" collected driftwood by leaving it in the sun on dry days when the outdoor humidity is less than 60%. Most Aquarium driftwood is kiln-dried, but the outdoor sun will work just as well. It is possible to build a solar kiln to dry driftwood, however there is a fire risk to be aware of and size/cost limitations may make this unreasonable. 
• Examples of hard woods that are known to be safe include, but are not limited to:
  • Mopani
  • Azalea
  • Ash
  • Apple
  • Cherry
  • Basswood
  • Beech
  • Cholla
  • Elm
  • Oak
  • Hawthorne
  • Madrone
  • Malaysian
  • Manzanita
  • Mesquite
  • Pear
  • Rosewood (Dalbergia spp)
  • Birch
  • Sycamore
  • Alder
  • Bogwood
• Examples of woods that are not recommended for aquariums include, but are not limited to:
  • Grape vine
  • Horse Chestnut 
  • Yew
  • Walnut
  • Pine
  • Spruce
  • Ivy
  • Lilac
  • Cypress
  • Willow

 

I know I have forgotten some things on this list, and if you have suggestions please let me know and I will add them as long as I'm confident in my knowledge of the material. 

Fin

[Guppysnail]

This is outstanding. I’ve heard so much about different materials over the years and I’m never sure what to think. 
I appreciate you taking the time to write all this out and share your education with us. 
THANK YOU 😊 

[TheDukeAnumber1]

@Sarina So glad you are here, I'll be following this thread for the learnin's 🙂

I would love your thoughts on PLA within the context of 3D printing & aquariums, as my experience with it goes against it degrading with water. I have tried over years to see if it degrades in aquariums/ponds and in soil with no noticable effects to structural integrity or even finish. I've read no name brand PLA's aren't always consistent, but even with no name tests I haven't noticed any degredation. Google diving has lead me to believe the idea that although PLA is bio degradable, this only occurs at high temperatures (145F-ish) and in the presence of certain bacteria. I am very interested to hear your thoughts on this.

Also I would love if you would take a look at my favorite filaments data sheet and comment. Thanks! -> Datasheet

[Sarina]

@TheDukeAnumber1 PLA is totally fine for use in aquariums as long as the application isn't structural. The problem with PLA is that it becomes brittle when exposed to any water pH higher or lower than 7.0, which means that structural components have the potential to crack and break. They usually break along the print seams, where the layers adhere to each other, as this is the weakest point in the structure and not all layers will adhere equally. There is also a lot of major differences in quality with 3D printing filament, and since this is a blanket resource with no specific brands, it's safer if I just advise people stay away from it for structural stuff. It would likely be okay for 2-3 years, but for most of us that keep tanks going long-term, that can pose an issue if we have a component suddenly break. I have seen PLA parts used to convert/modify bulkhead elements under pressure, and that's kind of the danger zone you want to stay away from with that material. Anything that could potentially cause a lot of water to end up on the floor needs to be made out of a different material. For decorative items, or items that won't ever be under significant water pressure, PLA is an inexpensive and easy-to-work with choice for 3D printing aquarium accessories. 

I hope that clears things up 🙂

[TheSwissAquarist]

@Sarina  kudos to you for researching all this! I can confirm that pine is definitely unusable in a tank. 😅

[memorywrangler]

On 8/16/2023 at 7:28 AM, Sarina said:

Lapis Lazuli

Do you know what the problem is with lapis?  I had great plans for lapis in a shell dweller tank...

[Guppysnail]

On 9/19/2023 at 1:18 PM, memorywrangler said:

Do you know what the problem is with lapis?  I had great plans for lapis in a shell dweller tank...

I asked a physicist friend of mine about lapis lazuli I wanted to put in my tanks. Apparently it leaches sulfur and loses its finish. 

[Sarina]

On 9/19/2023 at 12:18 PM, memorywrangler said:

Do you know what the problem is with lapis?

@Guppysnail is correct. Lapis lazuli dissolves in water and releases sulphur and it can also contain toxic minerals like asbestos and lead. In general, blue and green minerals/stones with a mohs hardness below 7 will usually dissolve in water and often contain asbestos, sulphur, copper, and other toxic minerals/metals.

 

[Sarina]

ADHESIVES ADDENDUM

I'm going to add some information about adhesives here, since I have come across a couple of instances where this has been relevant. 

Super glue:

• Any cyanoacrylate glue will fall into this category and is safe for aquariums. As with all adhesives, please use in a well-ventilated area or outdoors, and use proper PPE to prevent injury and illness. 
• Super glue cures by evaporating a solvent and hardening the dissolved polymer material, which is accelerated by chemical catalysts or water/humidity. 
• My favorite type of super glue is BSI (Bob Smith Industries) "Insta-Cure/Maxi-Cure" CA glue. It comes in several different thicknesses that are great for different purposes and the bottles/applicators for them are excellent. You can even get precision applicators that work extremely well for putting glue in hard-to-reach areas. Please note that BSI makes lots of adhesives, and any that are not in the insta/maxi cure family have not been tested to be safe for aquariums. 
  • The thin glue is great for soaking cotton, dry sphagnum moss (orchid moss), or areas of substrate that you want to solidify without adding bulk. This will soak into things via capillary action and cure extremely quickly, but the exothermic reaction can be substantial due to the fast cure time. If you use too much glue and the heat from the reaction causes it to cure too fast, it can turn white. This can be easily covered later, but something to be aware of, less is more with thin glue. Be aware that the exothermic reaction can and will burn you if you aren't careful, and it will releases gasses that can burn your skin/eyes/lungs. If you have a vapor fan, use it. 
  • The medium glue is great for sticking hardscape together without a bonding material like cotton or sphagnum moss, but takes a bit longer to cure. You can use a catalyst if you want, but I prefer to just use clamps and let it cure over time to avoid the white cast that is common on accelerated cures. You may be tempted to spritz some water on it to cure it faster: Don't. This will cure the exterior of the glue blob, leaving the interior encapsulated, which usually takes even longer to cure because now there is a hardened shell that the solvent has to evaporate through. You are better off just leaving it alone and letting it air-cure. 
  • The thick glue is what I generally recommend for gluing rhizome plants and moss to hardscape. It takes the longest to cure of the 3 options, however the thickness of it helps grip the plant and the slower cure time helps prevent the glue from overheating and burning the plant. Go ahead and dunk this one in water to cure it, the thickness of it helps prevent the gooey center problem that the medium thickness has, and it will help prevent your plants from drying out while waiting for it to cure fully. Catalyst is also effective, although you'll want to rinse the glued item before adding it back to your tank to get any catalyst residue off if you don't want to wait for it to evaporate. 
• Gorilla glue is terrible. This is just my opinion, but i feel like this opinion is warranted. If you're going to get any other brand of super glue, get the stuff in little metal tubes like this and stay away from Gorilla glue. In the model making community we joke that it's a rookie mistake to start with Gorilla glue because it's essentially the worst product with the most marketing hype. 

 

Epoxy Putty

• Please use PPE and in a well-ventilated area. Epoxy fumes are still toxic, even though the cured material is very safe. 
• Usually the way this is sold is in a flexible rod, with one part of the putty inside the other part so that all you have to do is cut a section, fold it a bunch to mix it, and then apply it to the area. It's very user-friendly and great for reef applications where heavy chunks of reef rock need to be stacked precariously together.
• Safe for aquariums unless it contains metallic compounds, as mentioned above. 
• Great for gap-filling and ensuring heavy objects stay stuck together, but is extremely expensive compared to other methods. Usually epoxy putty does not degrade or get brittle like super glue can.

 

Silicone 

• Use proper PPE and do not forget the gloves, this stuff is not fun to get off of your hands. 
• Can be mixed with sand, dirt, or other substrate material to coat foam, glass, and other areas where you want to hide construction materials/processes.
• Can also be mixed into a putty using substrate material to fill gaps in hardscape or create support, as a cheaper alternative to Epoxy putty. Adhesion to certain materials may be limited depending on the surface and how thick the putty is mixed. 
• See above for more info on silicone. 

[Guppysnail]

@Sarina  I bought these for my frog tank and hillstream loach breeding tank. I finally set up the breeding mound for the loaches last week. 
A few days later I started worrying because I remember you saying stay away from green and blue rocks. Now I can’t stop worrying about it. 
I tried to see what they were made of but it does not say. They are specifically for aquariums. But the biorb line is mostly focused on just decorative tanks. 
Do you feel at your best guess these are safe? 
Sone of the green rock are starting to get brown spots that do not wipe off. In all honesty though I cannot remember if the brown spots were there when I put them in. 

[Sarina]

On 10/5/2023 at 12:23 PM, Guppysnail said:

They are specifically for aquariums.

This is the key phrase. These are likely dyed or heat treated to get to this color. This is also likely a rock that has a mohs hardness of 7 or more, meaning quartz-based. The polished surface is a dead giveaway, softer stones kind of disintegrate when polished. 
 

Anything made specifically for aquariums is going to be safe in general, though. 

[TheSwissAquarist]

On 8/16/2023 at 4:28 PM, Sarina said:

Examples of hard woods t

Would the botanicals (e.g. leaves & co.) from the safe woods be fine in aquariums? 

[Sarina]

On 10/7/2023 at 9:29 AM, TheSwissAquarist said:

Would the botanicals (e.g. leaves & co.) from the safe woods be fine in aquariums? 

This depends on the type of botanicals, but generally yes most botanicals from safe woods/trees are fine when fully dry. Most leaves that are fully dried (and dropped naturally) lose any toxic qualities they would have when they are alive. It's actually a really cool function of evolution, when deciduous plants drop their leaves seasonally, the leaves become less or non-toxic so that detritivores, fungi, mold, and other functions of decay can consume them and convert them into nutrients that will end up going back to the soil. (Insert circle of life song here lol)

There are some exceptions to this, and I would not recommend using any botanicals collected from areas that could potentially be sprayed with insecticide or other chemical treatments. Avoid the leaves from evergreens, and any cones containing high amounts of sap/resin. Also avoid anything super sugary, like maple, which can cause severe bacteria overgrowth. You do need to know a bit about plants to safely collect both wood and botanicals, but you can find a lot of info on what's safe online, just based on what botanical suppliers sell. 

Some of my favorite botanicals that I have collected myself include:

• Alder cones
• Acorn caps
• Live oak leaves/twigs
• Magnolia leaves and seed pods
• Bracken fern fronds (I'm from Washington and these are everywhere up there)
• Apple tree leaves
• Nettle (a shrimp favorite)

If you are into blackwater like I am, botanicals are a whole deep dive and quite frankly I still feel like I do not know nearly enough about them. Here are some other botanicals that are commonly used in aquariums:

• Bamboo leaves
• Guava leaves
• Lotus pods
• Coconut husk/shell
• Casuarina cones and needles
• Indian Almond leaves/catappa leaves
• Senna pods
• Cocoa leaves
• Monkey pods
• Palm fronds/skeleton
• Cashew leaves
• Marupa leaves
• Locust pods
• Banana leaves/sticks
• Tingui pods
• Urchin pods
• Betel nut pods
• Sterculia pods
• Ceylon cinnamon bark
• Bael tree pods
• Hazelnut twigs/leaves
• Dimocarpus Longan leaves
• Jackfruit leaves
• Swamp fern leaves
• Red mangrove leaves
• Loquat leaves
• Mahogany pods
• Mango leaves
• Sappanwood pods
• Sapucaia seed pods
• Arjuna pods

I highly recommend this website if you're looking for botanicals. They have by far the best selection and source ethically. You will find most of the above available there and probably some that I have forgotten.