I was wondering, does the size of the tank affect the PH? I have three tanks. 55G Tropical, 20G Goldfish and 3G Betta. The source of the water is the same for all three. The 55 is planted but the others are not. The PH on the 55 is always at 6.8, the 20 at 7.0 and the 3 at 7.2. The...