How old is the tank?
When you say your water parameters are "perfect", that doesn't really give us much to go on...values are what we need. Algae blooms in response to an influx of nutrients (generally NH3, NO3, and/or PO4). Overfeeding and/or not rinsing frozen foods is often a factor here, as is overstocking the tank.
Lighting can make a difference too...algae favors the red end of the spectrum, so old lamps or the wrong lamps can contribute to the problem, as can too much intensity or too long a photoperiod.
What (if anything) has changed in your system (new additions, equipment, etc)?
How large is the tank?
How is it set up (sump, HOB, canister, etc.)?
What is your tank stocked with?
How often do you change out water, and how much?
Do you use RO/DI, tap, etc. to mix your water?
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