Daphnia is a pretty versatile food and can be fed to tropical, coldwater, marine fish, and turtles. It's high in protein and aids digestion so it's an ideal food to keep a stock of, and especially beneficial to offer fish that are recovering from illness and may have been off their food for a while. The amount you feed is dependant really on how many fish you have in your tank and of course the variety and size of them. Fish will over eat if you allow them and this often causes water problems as excessive waste matter can quickly pollute the tank. In general, the rule is to feed your fish no more than they can consume in a period of around 2 minutes, and not more than twice a day although once a day really is sufficient. Always error on the side of caution and feed too little as opposed to too much, your fish will suffer no harm from being slightly under fed. Daphnia float so if you do accidentally over feed, simply skim the water surface with a fine net to remove the excess. (Never leave it there, it can pollute the water). Freeze dried daphnia can be used as the staple (main) diet for your fish or as an occasional food/treat. Remember though if using as a treat it still counts as a feed and should substitute rather than supplement that meal time offering. Also consider the needs of your fish, as this food floats you may find your surface feeders hog the lot before your bottom feeders or shy fish have even realised it's feeding time.