10-15% water changes weekly ought to be adequate, but...
...alot of absent info here.
What do your water quality tests say?
How old is this set up?
A bit of black algae? How much is a bit?
What kind of algae? Have you tried to ID it? Take a picture and post?
Four fish? What type? How big?
What else have you got in the tank? Inverts? Corals? Live Rock? (more than just fish contribute to a bio-load).
Are you skimming?
Cheers,
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Mark.
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