If you are refering to a suppliment for nutrition, that would greatly depend on what you feed them. Some good marine additives are Selcon, Zoe and Vita-chem. I personally have found concentrated garlic to be of benefit as well.
If you are asking if there is something to be adding to the water, then IME
the best thing you can do is maintain good water quality. Of which the easiest is usually regular water changes. Nothing more