Select feeding is hard but it can be done to an extent with a mixed tank. Feeding a high quality pellet or flake with a mix of proteins and veggies for a base is a good start.
I wouldn't worry much about it unless its a really specific fussy eater, ive heard stories about too much protein in strictly herbivorous fish can cause problems and in cichlids also increased aggression. Just feed a variety of food, a good flake and/or pellet, algae wafers, some frozen bloodworm, daphnia and brine shrimp with some fresh veggies mixed in. Most fish will enjoy a balance, what fish were you thinking of keeping or dont you know?
Fish die belly upward and rise to the surface. It's their way of falling.