Algae in Pond 0No3

[Talkinggoat]

I have a small pond that's been established for over a year, with a serious algae bloom. I normally combat it with mechanical filtration, but I decided to perform a water test, just to see what was up. I found the ph was very high, at 8.2-8.4. I don't know why, because the water I use is not that high and there is nothing in it, like concrete or limestone. The Ammonia was low at 0.25, No2 at 0, No3 at 0 and phosphates at 0. What could be feeding the algae? I thought maybe the algae is soaking up all the No3, so I'm afraid to put any kind of plants, for fear they might starve.