It caused by sun lite and mostly phosphates in the water.
Are U using your garden hose to fill it?
If so that is your house water which if unfiltered has phosphates perhaps also the most likely culprit is the rock itself, it has phosphates.
U don't think a landscaper understands this stuff do U when they pop a rock in with a little pump.
I'll deal with this all the time in the aquarium biz.
two things filtered water and barley straw will decompose adding hydrogen peroxide to the water, killing the algae.
Uptown girl your use of chlorine puck will not balance that pond water at all, you may want to think about adding pond plants which will create it's own eco system and naturally eliminate algae
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Nova Water filters
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