All parameters are not fine then. Fish don't die for no reason. What is the source of the algae, is it overfeeding, not using RO/DI water, overstocked bio load or too many fish, poor circulation, direct sunlight, poor lighting or wrong spectrum, no biological filtration, wrong type of filtration for application.........
Algae is a sign of high nitrates for one thing which can be directly linked to several of the things above.
How old is the system, what size is it, was it cycled properly, does it contain messy fish like triggers or puffers......