What Can Happen if a Goldfish Is Overfed?

An excess of leftover fish food breaks down and clouds the water. You can feed goldfish two or three times a day to encourage their natural feeding behavior but always remove any uneaten food with a net after five minutes. Decomposing fish flakes form a layer of sludge in the substrate and on tank ornaments; it pollutes the water, looks unsightly and is toxic to your fish in the long term.

Overfed goldfish produce a relatively enormous amount of waste. They are naturally voracious eaters, and if you offer too much food they will almost certainly eat more than they need and still go back for more if its left in the tank. Goldfish have a rapid digestive system; look for trailing strings of feces still attached to the fish's body for an indication that your fish is over-eating. This excess waste ends up in the bottom of the tank and leads to poor, even deadly, water quality.

An early indicator that your goldfish is overfed is an unpleasant odor coming from the tank. Decomposing matter makes the water smell bad and a mix of uneaten food and fish waste also creates the perfect environment for bacteria to breed and weaken your fish's immune system.

Poor water quality quickly has an effect on your fish. Healthy fish have a slime coat that protects them from bacteria and disease. If this protective coating is weakened from living in dirty or polluted water, the fish becomes susceptible to infection from common parasites and conditions such as white spot, fin rot or bacterial gill disease. Bloating, raised scales and a swollen abdomen are symptoms of swim bladder, which can also occur as a result of overfeeding.

Green or brown slime on aquarium glass is algae. Algae are simple organisms that thrive when exposed to excess nutrients left in the water from overfeeding your fish. Although a small amount of algae isn't bad for an aquarium, it is unsightly and if left unchecked can choke aquarium plants with thick, stringy plantlike matter. Nitrate, silicate and phosphate levels are much higher in polluted water caused by decomposing fish food, and these three substances also contribute to an excess of algae.