6 Answers

Growth of algae.

Maybe it's envious?

Needs a good clean.

https://www.the-age-of-aquarium.com

Time to clean the aquarium I would think before the fish die if they have not already.

Algae…yes... as a result of an imbalance between uric acid producers and uric acid users. Fish produce uric acid and plants absorb uric acid. Some species of fish produce much more uric acid  than others, such as gold fish and most carp varieties. Goldfish produce so much uric acid that many other species will die off in tanks with gold fish even though they may be socially compatible.

Changing water frequently will help control algae. Barley grass will end algae build-up and will not harm fish. I use barley grass in a koi carp pond.