You can use ABS to an extent in a tank. When ABS heats to a certain temperature (I think it's 60 or 70 celcius or so) it releases a toxic gas. Usually when using ABS in a tank it's kept to fittings and such. I think that's the reason why PVC is pottable and ABS isn't (pottable meaning safe for drinking water).
With the ammount used on a tank I just use PVC. It's not like you're plumbing an entire house with the stuff
