2500-6 is 100-what?

first, is it even safe to use a PH lower made for a pool? if so it says 6oz for every 2500 gal. which is how much for 100 gal tank? and would my water stay balanced? also after adding ferts..

That would be 7ml for every 100 gallons. I'm not sure about using that stuff however. You can get a pH kit for like 20 bucks that has up, down and a tester.