What things are toxic to freshwater fish?

[Kaleb Heiermann]

I was thinking about putting in a metal or maybe plastic scale plane model in my aquarium and then growing moss or maybe Monte Carlo over it. Would that be in anyway harmful to the fish? And what are somethings to watch out for? I’ve heard koi and copper don’t mix so I was curious.

 

[Daniel]

@Bill Smith has Star Wars dioramas in his aquariums, but I think those are plastic.

[Kirsten]

It's true, copper can be harmful to many aquatic species, especially snails. Since most metals may be alloys of some kind, or may be plated or coated over a cheaper base metal (not to mention the risk of rust or corrosion when immersed with good air flow), I'd avoid metal if you can. One exception is the lead plant weights sold for aquariums. Apparently those are fine for fish! But you probably won't find lead in too many commercial items these days.

Plastic should be fine, though. Some people get worried about the paints, but I haven't seen anything conclusive about it. People use plastic plants, skulls, pirate ships, Spongebob pineapple houses as decorations all the time and as far as I know the fish are happy 🙂

[Fishdude]

I had a plastic skull in a tank during Halloween and started having fish die off until I removed it and changed the water. Could be unrelated, but I lost quite a few tetras during that time so...I dunno!

[gardenman]

17 hours ago, Kirsten said:

It's true, copper can be harmful to many aquatic species, especially snails. Since most metals may be alloys of some kind, or may be plated or coated over a cheaper base metal (not to mention the risk of rust or corrosion when immersed with good air flow), I'd avoid metal if you can. One exception is the lead plant weights sold for aquariums. Apparently those are fine for fish! But you probably won't find lead in too many commercial items these days.

Plastic should be fine, though. Some people get worried about the paints, but I haven't seen anything conclusive about it. People use plastic plants, skulls, pirate ships, Spongebob pineapple houses as decorations all the time and as far as I know the fish are happy 🙂

"Lead" plant weights these days are typically made from either magnesium or zinc. I would avoid putting anything metal into a tank including some stainless steel as not all stainless steel is truly rustproof. There are different grades of stainless steel with varying degrees of corrosion resistance. Some people who bought stainless steel appliances are finding out that the stainless steel in their appliance is less corrosion resistant than they'd assumed as it's rusting. There are some very angry consumers out there with that issue.  If you have something metal you really want to put into the tank, seal it first. A good coat of a clear polyurethane should do the job. If it's small enough, dipping it in the clear poly and rolling it while submerged to ensure every surface is coated is smart. A spray on coating is the second best option, but getting poly on every surface is a challenge. Then you want it to dry for at least double the cure time stated on the poly container. A good coat of polyurethane should be tough enough to survive even the most determined pleco scraping away at it for years. Poly comes in everything from flat to high gloss to, so you have options in how the finished product looks. Higher gloss typically means harder finish also, so if you want the hardest possible finish, you go with the higher gloss. 

[Fish Folk]

On thing to consider is using activated Carbon as part of your filtration in a tank where you intend to include unnatural pieces of hardscape. That can draw out certain unhealthy agents. Unfortunately, it may also draw out desired fertilizer in your planted tank (e.g. you want to grow something like Monte Carlo on your hardscape item). Here's a video that clearly lays out the caution side of using carbon in a planted tank.

I recommend using a resin model designed for use in aquariums. Here is an example you can buy. It will cover over with natural green algae in time if you just let it go. There's a bunch of similar options available for sale online made from resin that should minimize risks. Enter "resin plane model for aquarium" into a Google Images search, and you'll come up with a number of similar options.