High pH

[sperry]

I have a 10g QT / chaeto growing tank that I pretty much ignore other than add RO top off water to every once in a while. It has a small HOB filter, a stock nano cube pump in it, and 2 24W PC 50/50 bulbs. I am going to need to put a new fish in it this weekend so I was testing the water last night and the pH was off the scale 8.8+ the API test turned a dark purple that I have never seen. I thought maybe it was due to the chaeto so I tested again this morning before the lights came on and it was still off the scale. I did a 3g water change which lowered the pH some but it is still above 8.4 I know I can add vinegar or carbonated water to get it down but what is making it so high? I dont add anything to this tank. It doesnt have sand or rock in it.

 

[neuroslicer]

what's the alkalinity and Ca?

 

[sperry]

Didnt have time to test it before work this morning. Left the lights off all day still thinking it was the chaeto and the pH is down to 7.8 now. Ca-300 Alk-1.5 currently.
I just wanted to keep some pretty fish, didnt know I was gonna have to be a chemist, plumber, electrician and all around handyman in this hobby. LOL