Lowering ph

I have a problem,(I guess if I didn't I wouldn't be asking a
question!),anyways,I have a 90 gall heavily planted freshwater community
tank that's been set up for 10 years.Because I've always had a mild amount
of algae,I was doing large (70 %)water changes weekly to try to eliminate
the algae, but in the last 6 months it's gotten worse.(I think it's
cyanobacteria)I don't think I over feed or the tanks over populated.
I started adding KNO3(potassium nitrate)to increase the nitrate and have
reduced the water change to 5 gallons a week and it seems to work but now
I've noticed the ph has risen to above 8!.The tap water is about 7+/- .To
confuse matters even more,I also have a well established 50 gall that I
haven't changed my normal routine(water change,feeding ,fishload,ect...all
the same as usual)and the ph in that tank is also over 8.


So my questions are would KNO3 raise the ph?
If it's not the KNO3 what could it be?
What is the best way to lower it and keep it down safely?