Water quality issues, temperature changes, genetic factors, etc.

1. Goldfish’s eyes are completely black because the water quality has deteriorated. If a large number of bacteria breed in the water, the goldfish’s eyes will be contaminated and turn black.

2. Goldfish are more sensitive to temperature changes in the breeding environment. If the water temperature rises, the goldfish's eyes may become completely black.

3. The discoloration of goldfish is greatly affected by genetic factors sooner or later. Some goldfish are born with completely black eyes. This is a genetic characteristic called grape eyes, which are common among goldfish. Gene shape and induction by environmental factors can lead to completely black eyes, but when environmental factors change, there is a chance that grape eyes will return to normal.