I have four goldfish in my tank and one of them always is at the bottom of it and I think its tail is broken or something because once he got stuck in the filter

I think I might be over feeding my fish, and there are food left at the bottom of my tank I heard someone say that it could turn intp toxi (sorry not good at spelling) what should I do?

Black spots on goldfish is a result of poor water quality being that the ammonia level in the aquarium has spiked. When the ammonia level in your tank increases it causes black spots to appear on the goldfish body and black tips to appear on the tail, pectoral fins and dorsal fins. These black spots are a result of ammonia burn. People should think of ammonia like acid, when the ammonia is extremely high it cause ammonia burn, just like acid would do to human skin. It causes discomfort in your goldfish and high levels of ammonia can kill your goldfish. As I've mentioned before people really need to take care of their goldfish properly. If you have an aquarium or goldfish bowl, you need to have water test kits around. This is a necessity if you plan on keeping your goldfish healthy and living a long life free of black spots.

Now don't mistaken black spots on goldfish being ammonia burn all the time. Goldfish do change colour from grey to common colour orange so that may be the case as well. Even though your goldfish aquarium water may be crystal clear, doesn't always mean that your water quality is perfect. Clear water could also contain high levels of harmful properties in water chemistry. Never rely on clear water being safe for fish. Check your water quality often to ensure that you don't have black spots on goldfish and ammonia burn to happen.

Test often, perform regular maintenance such as water changes and have equipment working properly will be the first step to proper goldfish care and eliminating chance of black spots on your fish called Melanophore Migration... hope this helps! :)