I used AlgoPond Direct and the thread algae died. But they have come back and phosphate is not detectable.

This last statement is probably right according to the user’s measurement, but factually incorrect. Phosphates are always present. They are not only in the free flowing water but also in the pond sludge, as mineral-based components in the pond water (though not indicated through PO4 tests) and there is a permanent replenishment of phosphates. Furthermore many algae species can store phosphate in great quantities, some of them up to 100,000 times of their own requirements. This depot is for “hard times”. It only needs surviving algae spores, which can grow again. There is no certainty that a single application of AlgoPond Direct keeps the pond free of algae for the whole season. This would be good but can’t be guaranteed for biological reasons.