i would wait to buy that until someone more knoweldgeable pops in.
I'll try to help with the little that i know
1. Ph can be affected by CO2 ( i think) like if you have a lot of people in the room the ph goes down becuase of everybody breathing, houses closed up in winter and ph can go down. SO id there a window near the tank you can open to get better air exchange on the tank? that may help
2. Can you measure your alkalinity ( KH/DKH/ALK) those are different ways of saying mostly the same thing but different test kits read differently.
3. Measure Calcium - those two things go together and balance like a seasaw to work in our systems. They also can have an affect on the ph in a tank.
4. What kind of flow do you have in your tank.
All that said I dont think you ph is too terrible
"we're going to need a bigger tank..."
