Lowering ph

I'll try to be a little more detailed in my info(I didn't want to overload
the newsgroup)The carbonate hardness is 71.6
The general hardness is-17.9
Nitrate-0 That's why I think I have BGA(cayanobactiria)
Phosphate-0
Iron-No tester for iron
Ph-8.0
Stocking level are medium to low...4 adult angels,4 3
inch botias,and 2 otto cats.
From what I've read LOW nitrates cause BGA,and normally I do 70 gallon water
changes per week(I thought it was getting rid of the algae)but that may have
contributed to the algae problem!

The tank itself is heavily planted,it did have 40 watt flo bulbs on a timer
for 12 hours a day but I reduced it to just 2 bulbs 12 hours a day to reduce
the algae.There is no gravel just play sand so its easy to keep the tank
clean(just stir it up with the gravel vac).
I get my KNO3 at a local hydroponics store but it's pretty expensive,if
there's a cheaper place I'd like to know.

Thanks again for the informative answers,you people are the best!