Alge and PH problems in a 50gl res

hey guys i need some advice on what to do with a 50 gallon res that has a float valve running from a small boy fillter. My tap water comes out at 140ppm with a PH of 7.1. I've had the res for about a month and i only use it for fresh readily available water at room temps but my PH after it sits goes down to about 6.7. After i add my nutes my PH goes down to 5.2. I feel that its due to something building up in my res. So my question is, is there anything that i can add to my 50gl res to keep the PH stable. Ive heard of H2O2 added is this a good idea and if so how much of what do i add?

most fertilizers lower ph. That seems a bit extreme though.

do you SEE any algae/froth or feel any sliminess on the sides?

I use pool shock for my rez