Well...they go long periods of time when all they will eat is fish. I was feeding them just fish for a while...but then I realized from reading posts on here how unbalanced that was so now I have been trying to alternate with worms and I went out and bought some pinkies the other day...they get picky of course, and all they want is fish...but I am going to try mixing pinkies and worms in with the fish and so forth. I think I will go ahead and buy the supplement so that I have it in case.