Maybe FA?? Sure experts will chime in, but if you have that in there I think you could put some hydrated lime in it and it will kill everything, plant, bacteria, etc. That would clean it up and I think within a week the pH would be back to normal. I had read where you can do that before you stock it or anything so you are starting fresh. Once it gets to 6' you won't have as much trouble with the plants like algae. Just some thoughts from what I've read and learned and wished I would've known when we built ours! Congrats on the new addition!!