44+ Brown Spots On Goldfish. Brown or black flat spots and patches on goldfish are a sign of ammonia burn from spiking ammonia levels. Brown spots on my goldfish?
Fish Disease And Treatment from animal-world.com
Oxidation is simply the natural. If your pond does need a complete clean out, good preparation a pond is a more natural environment for goldfish than an aquarium, but the fish numbers are usually much higher than in a natural pond or slow. If your goldfish went black and then died, the black/brown markings would have been ammonia burns and the goldfish would have died from ammonia poisoning.
This is actually one of the quite common goldfish issues.
Goldfish tend to be messy and dirty fish. Pet goldfish can quickly destroy a planted aquarium. However, most goldfish grow to only half that size. The culprit for this discoloration is oxidation, which happens naturally the longer cauliflower is stored.
