Last day of spring: sunny, cloudy, in the 70s!

Poor water quality (unhealthy for fish) or lots of light (healthy for the fish) can both encourage excessive algae growth in a tank. Meaning that by looking at a tank’s surface algae you can’t determine the health of the system. More information is needed. In both cases, the surface algae itself is not harmful for the fish, so even if you fix the water quality problem but don’t remove the algae, you could have an algae covered tank in a moderate amount of light that’s still healthy for the fish.