Hi, since this is a casual game, I have moved your post over to the Casual Games forum.
I found some iPad and iPhone instructions for this part of the game; I don't know if these would be the same for the Android. What it says is to tap on the fish tank to zoom into it and drag the fish food from your inventory onto the fish tank to feed the fish.
If this doesn't work, I'm not sure what to suggest. Maybe someone else will be able to help.