I just made a plexiglass tank lid that is perfect, but I heard from someone that plexiglass emits a toxic gas that could be leathal to fish, I was wondering if that has any truth to it?

I do know that Plexi gives off gas, as to what level for the fish I dont know hun. I know that Plexi will melt if a light is to hot for it I have done it my self.

Thats why most tanks that arent glass are Acrlic tanks....

Hope this helps

i used plexiglass for a short bit, no problems though.

Plexiglass is a particular trade name. Lexan and various acrylics are standards that are used for tanks. There are certainly some brands of plastics that give off less plasticizers and such into a tank.

Plexiglass is a particularly flexible plastic - it will tend to sag over time - so you will want a center support for the lid. Try to make sure the plastic does not hang into the water. I also would recommend that it be used only in a set up where there is an opening at the back of the tank where the filter and other equipment enters the tank system - and letting lots of air exchange. You do not want a "sealed" system in the case of Plexiglass.

LExan is a particularly strong and stiff plastic material. It has less platicizers in it and thus is less of a potential hazard in a tank situtuation. It is suceptible to sheer, though.

Many plastics can release the majority of their "toxins" in a relatively short time. Just like you would want to "season" a plastic tub or pool before use, you can soak the plastic you wish to utilize in the tank for a few weeks and in most cases, be confident that 99% of the platicizers have been removed.