Okay, PH is one of those things that do get confusing. I'm not sure I fully understand it and I've been at it a long time. But the big thing is that it changes and it does read different at times. One big thing you've spotted is the way our hobby grade test kits leave some doubt. When you have colors printed on paper, there is room for error. When we look at those colors under different light it adds more room for error. Go to a store and look at a color under the store lights and then it may be different when we get it home.
So I start off with doubt that what I read is going to be exactly right and combine that with the fact that water in nature changes constantly. Think of a lake out North in the piney woods where there are a lot of pine needles. When a heavy rain comes along lots of those needles go into the water and the lake may become more acidic. Or during a dry time, the Colorado river may swing alkaline or more toward acidic as debris is flushed out or washed into the water. Tap water has gases in it and as those gases disperse into the air, the PH changes. When you add it to the tank, there are minerals in the tank which may change the PH. Limestone tends to push PH up while wood tends to push it down. The PH changes and the fish are used to it. Happens all the time so I tend to downplay the importance of PH. I know it is often one of the first things mentioned in the proper care of fish but I don't find it nearly as big as the mineral content of the water.
Adding CO2 to a tank often forms carbonic acid and that pushes the PH down but the fish are not bothered. I keep African cichlids who might find 8.0 PH in Africa but they have no trouble spawning in 6.8 when I run CO2.
My advise is be aware of PH but generally ignore it!
I should not say to ignore it! Be aware of it but only worry if it is changing as that would be a first indicator that there is something going on in the tank. A little this way and that is no alarm but if it continues to drift you may want to ask what's up?
Last edited by PlantedRich; 12-20-2015 at 02:07 AM.