Interesting - why not? Might look into small pond pumps/filters - I have one in an indoor stock tank but find that its too noisy- vibrates too much - fish dont like it. So, I too am looking for other options - something that could work.

One thing about using an old bathtub - every molecule of soap residue has to be removed bc it harms the fish, even very small amounts. The stuff commonly used to remove soap film could be hazardous too so Im not sure how you'd address that exaclty.

Caveat: I dont know anything about tropical fish - pond comets is about all I've ever had.

Hi
I assume it would be located somewhere indoors?? You could use an air powered sponge type filter. Would require only a single airline with the air pump located anywhere.
Do you intend to add a substrate ,plants other fish??
gary

Yes it can be done. You would need to remove all soap residues as mentioned earlier. You would want to make sure there were no chips in the paint as it would cause rust. I personally would run a canister filter using the drain and faucet holes in the tub. As far as the temp is concerned, the water will be 5-10 degrees cooler than the room due to evaporation. Let us know what you decide, if you end up converting it, please post pics. Good Luck!