I was actually in this situation earlier this summer, a neighbor boy had brought home about 10 baby bass from his lake house because he knew I liked fish...(gee thanks kid). I had to care for them for three weeks before we could get them back to their lake home. I set up a bare bottom tank with lots of artificial plants and a cycled sponge filter and lots of java moss (a "lakey" environment if you will). I tried feeding these guys everything from flake food and cichlid pellets to betta bits, frozen foods, etc. The only thing they seemed to figure out was food eventually was freeze dried bloodworms. Even then I still lost one to what I'm assuming was starvation. I highly recommend you return the bass to it's natural habitat ASAP, they aren't meant to be kept in fish tanks.