Until you research level 3 of fish environment, the more exotic fish will not survive. Fish food research won't help the babies survive; it just lets you feed the fish less often. The best thing to do is to breed and sell lots of fish that will survive at your current level of research in order to build up some cash. At level 2 of fish environment, you should be able to find several fish that will bring good prices. If you haven't purchased the second tank yet, I'd recommend working on that next, because you will be able to make money faster.