A short time ago is was very normal to bring all sorts of treats along with as your snorkeled to feed the fish. Things like bread crumbs, frozen pees and even common fish food were all used to lure the fish closer and interact with them.
Many snorkeling destinations now prohibit such activities. Some may still allow it – but it is not recommend. The reasons are pretty straight forward.
First is that it is not the fishes natural diet and eating behavior. This foreign behavior causes havoc on the reef and its fragile ecosystem. This may seem like an insignificant detail – but it has cascading effects. When you feed the fish a foreign diet, they begin to rely upon it. They no longer feed naturally off of the reef. In most cases this is algae that grows on the reef. This algae becomes dominating and invasive if not naturally pruned and thinned by the fish eating it.
The unnatural feeding also causes conflict amongst the fish due to size. Larger fish are known to drive out smaller sized fish in order to have better opportunities at consuming the foreign food.
Another reason is somewhat similar to why you are not supposed to feed the bears when camping in the wilderness. The bears become reliant on the food and will stroll into camp looking for it. They become destructive and violent from their size and desire to find a meal. Many snorkelers often forget that fish are actually a wild species and some even cary very sharp teeth. An innocent attempt to get some food from you may end up leaving a very nasty bite wound on your hand or fingers.
It is understandable that you want to get closer to fish and enjoy their beauty. The next time you are in the water snorkeling around a reef, try paying attention to the sounds of the water. Listen for that snap, crackle and pop sound (sounds like pouring milk over cereal). That is the sound of reef fish feeding naturally off the algae on the rocks.