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i found a whole bunch of FIMO brand clay (see fimozone.com) from when i went through this period that i thought i could become a great clay figurine sculpture and clay bead designer (obviously it did not work out). Anyway, now i was thinking about creating some custom things for my freshwater betta tank, you know, so he will be all ..unique..original..and full of bright colors..
any i was wondering.. would this be safe? I cant remember whats in it.. whatever polymer is... i know i shape it and put it in the oven for like 20 minutes and it smells real bad if i burn it.. thats about all i remember.. would this be deadly for my fish? could i buy some .. i dont know.. stuff to coat it with? what are normal aquarium decoration/ ornaments made of? i just want to be artistic..

Probably not a good idea. Polymer clay doesn't usually sink and some will not survive in water long term. Check out this experiment: http://www.garieinternational.com.sg/clay/shop/porous_test.htm That person does claim to have left objects floating in the tank without dead fish. However they only say over days or weeks they haven't seen dead fish. Unless success can be measured in years I'd still be skeptical.
I also see at least one person says it is potentially toxic. Scroll down to Miscellaneous Tips on this site:http://www.katopolyclay.com/tips.html Unable to find any reports of polymer clay being used for years in a tank with no problems I wouldn't suggest just putting them in without some preparation.

You could probably coat it in epoxy or aquarium silicone(a bit messier and less fool proof but easier to find and cheaper) but then it has an even higher chance of floating since it's sealed from absorbing the water. You'd have to glue it down to something unless you want a floating decoration. If you really want to it could probably be done safely but would require a little work.

thank you! that was a lot of useful information! i didnt even think about them floating. Maybe I could figure out some way to wait them down. i definately wouldnt put foam in the middle.. thanks!

polymer clay is a porus material that can leech nasty things into your water (such as the ammonia used to keep it soft, or the dyes used to color it). Also, the water will degrade the clay over time (a few months) making your piece fall apart, or turn slimy and gross.

You can coat a polymer clay piece in epoxy resin (must be epoxy, not a glaze or paint-on coating) to make it more aquarium safe.

Hoping that there must be a word for everything I mean...

An idea for unique aquarium decorations, would be to buy some of those flat stones you find at the local discount stores (arts and crafts department) and a tube of aquarium sealer. You can stack the stones with the sealer between them, creating interesting caves, castles and tunnels for the fish. You will have to tape them together, or prop them against something solid, till the sealer drys between stacking more stones, but it works, and the "creations" won't harm the fish. Just make sure you leave enough room for the fish to swim through so none can get trapped. I have done this, and it makes for some interesting tank decorations.

Oooo... that's a great idea Two Tanks!

That would make great novelty decor for a fantasy-themed tank.

Hoping that there must be a word for everything I mean...

I'm glad you liked it! Just make sure it is all good and cured before adding what ever you build in the tank and use plenty of sealer. The only drawback, is that you will have to remove all your creations if you have to use meds in the tank - it will discolor the sealant (learned this the hard way). You can have a lot of fun with this project so long as you don't build anything too large or heavy where the weight might cause it to come apart.

sounds great! i do want to make some caves.. now i have to find the right colors.. is aquarium sealer sticky and messy? this sounds like a weekend project..

This would be perfect for my office tank, that has too little light for ANY plants grow

And yes, aquarium safe silicone can be very messy. Definitely cover your work surface with newspaper or something, and work in a well ventilated area with a fan blowing for plenty of air circulation. You might also want to use gloves (though I don't) because some people have skin reactions if you get any on you. The great thing about silicone is that if you mess up, you can usually peel the sealant off once it's dried a little.

Hoping that there must be a word for everything I mean...

Yes, aquarium sealant is pretty messy. Get you a big roll of wax paper to cover your work area. The stuff won't stick to it - easier to remove at least. You could end up with a mess if you use newspaper or very thin plastic. I want to see these projects when you finish!

Also wear throw away gloves. If you get it on your hands, it can be really hard to get off - learned this the hard way! You will need a ventilated area, turn on the ceiling fan if you have one, or if the day is warm, work outside.

i have to go out of town this weekend, so i wont get to it till next weekend, but i will post pictures!!