I have heard of their epoxy garage floor coatings but after reading your post went to google and found they make at least one potable water safe epoxy called Devoe Bar Rust 233h. Was that the specific Devoe product you were considering? They appear to have a very wide range of epoxy and epoxy based coatings.
I can't find the link again but I did find one very low price for a 1.25 gallon kit. Even at 80% solids it would work out to be pretty cheap. If you can get it locally and avoid shipping charges it might make for a cheap plywood tank sealant. I assume you're considering this for a plywood build? Of course I can't say for sure it's as flexible as other epoxies or if it's absolutely fish safe. I'm not one to assume potable water safe means fish safe.
As far as cheaper options, there are a lot of very affordable epoxy resins out there nowadays. The US Composites and Max ACR both distribute inexpensive epoxies that have been used successfully on plywood builds.