Materials Science in Reference to Aquariums

[Sarina]

The following was originally posted in Photos, Videos, & Journals, but I have received a request to post it here. I hope it is helpful 🙂 

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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 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

(Original post exists at this link)

[Sarina]

Whoops I nearly forgot about this part:

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. 

[madmark285]

On 3/1/2024 at 1:56 PM, Sarina said:

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.

Big thanks for all your work.

Specifically for this one, the company Gorilla Glue makes different products including a version of super glue.

But what about their main product, the moisture activated polyurethane adhesive? The SDS (?) data sheet says it is toxic to fish. But many other have said that once fully cured (~ 1 week), it is inert and safe for aquariums. 

My opinion, moisture activated polyurethane works better than silicone when mixing with substrate for hardscape projects. Mix together with damp substrate then cover it with regular substrate as it cures.   Here is an example of a cave I made. I used a balloon filled with sand as a mold, a white sand & polyurethane and cover it with gravel.

[madmark285]

On 3/1/2024 at 1:56 PM, Sarina said:

Epoxy Putty

I believe J-B Weld WaterWeld epoxy putty is safe for aquariums. 

[Sarina]

On 3/4/2024 at 3:06 AM, madmark285 said:

The SDS (?) data sheet says it is toxic to fish

I would refer to the SDS for safety in this case, and I would believe them when they say it is unsafe for fish. Polyurethane is an umbrella term used to describe composites that could contain a myriad of different materials/chemicals, and any polyurethane that specifically reacts with water is likely going to degrade with constant exposure to water, which could release carcinogenic and toxic compounds. You may not notice an immediate reaction in your fish, but it could shorten their overall lifespan, increase the chances of cancerous tumors developing, and shift the balance of bacterial colonies to make your fish more susceptible to infection/disease. As hobbyists we do always have the lab equipment necessary to study the effects of different materials on our fish, so in general it is recommended to rely on the SDS sheets to tell us whether or not something is toxic to aquatic life. 

[madmark285]

Thanks for your reply. What I may do for future projects using this glue, cover it with silicone and whatever subtrate you are using. 

[Sarina]

On 3/5/2024 at 11:33 AM, madmark285 said:

cover it with silicone and whatever subtrate you are using

Pretty much, yes. You can also mix clear, aquarium-safe silicone with the substrate you're using and mold it a bit like clay. 

[madmark285]

On 3/5/2024 at 12:39 PM, Sarina said:

You can also mix clear, aquarium-safe silicone with the substrate you're using and mold it a bit like clay. 

FYI, from the mold making world. Mixing in a little vegetable glycerin to the silicone will help curing thick application of it. Glycerin is a humectant which attract moisture required for the curing of silicone which you most likely already knew.

Of course that raises the question, is Organic Vegetable Glycerin for skin care safe for fish? 

[Sarina]

On 3/8/2024 at 10:21 AM, madmark285 said:

is Organic Vegetable Glycerin for skin care safe for fish

Theoretically yes, but when I tried this trick I found it to be harder to mold into shape as it changed the viscosity, and it didn't stick together very well on vertical surfaces when the substrate is mixed in, so I don't recommend this for the specific application we are talking about here. The silicone itself never ends up being as thick as it would be when making molds, since it is just being used as a glue to hold pieces of substrate together. 

[madmark285]

Nice to see someone else creative with materials! So if making something a couple inches thick with silicone/substrate, will the silicone cure in the middle? Should you use damp substrate to provide moisture?

Something else I want to try, mixing spray foam with substrate. Of course that would be quite messy but curious if you could build structures with it. 

 

[Sarina]

On 3/8/2024 at 11:39 AM, madmark285 said:

will the silicone cure in the middle?

Yes, I have never had issues with the silicone curing in the center. Aquarium safe silicone is the stuff in a tube, not a two-part, so you can easily tell when it's cured because it will no longer smell like vinegar. It can take anywhere from 24-72hrs for a full cure, I recommend leaving it alone for a minimum of 72 hours just to be on the safe side.