Originally Answered: Why is the water in fish tank harder than my tap water?
In many ways, CO2 injected tanks are a different breed from a regular tank. Many people who know their stuff when it comes to a regular tank will find that they have to "break some rules" when it comes to a gassed tank.
The water coming from the tap has a higher level of dissolved CO2 due to partial pressures. That is why it is showing you a 6.6. If you let it sit for a while, you should actually see that change as the dissolved gases reach equilibrium in normal atmospheric pressure.
I am a bit confused here tho. Your question asks about hardness, but the explanation of your question only addresses pH concerns.
You have carbon injection? As in CO2 injection? What concentration are you running? And equally important, what is the KH in that tank? What is your lighting like?
If you are dosing Potassium, as in using Potassium Chloride, you have one of the three Macros covered. Nitrate is a bit shaky but usable on a gassed tank, but what are you doing about Phosphorus?
Since I assume that your question is actually pH related, not hardness related, all of these can come into play because if any of the Macro Nutrients, light, or carbon are deficient, you will have an imbalance. (You appear to have micro nutrients covered, so I am leaving that out of the picture for now)
The first thing you should probably do is determine the actual pH of your tap water after letting it sit out for several hours. That will tell you your starting point.
You also want to know the KH of the tap water and the KH of the tank water for a comparison. If the tank has a higher KH than the tap water, something is 'dissolving' in the tank.
Artificially lowering the pH with Blackwater Extract (tannins) is just chasing pH and trying to treat the symptom, not the cause.
My 100g planted tank runs at a KH of 4 with a pH of 6.6, giving me about 30ppm CO2, which it turns out (oddly enough) actually matches up with my drop checker.
If you are injecting CO2 and getting a pH over 7, something isn't right. Get those KH numbers and lets get this figured out and solve the problem.
[RE: The Added Info]
Without really knowing how much CO2 there is, guesses are really hard.... Getting the true pH of the source water is key here. That said, there is a distinct possibility that the KH is high enough and CO2 input low enough that you simply aren't seeing a drop in pH.
Another possibility is that as the CO2 introduces carbonic acid into the water, that acidity is reacting with something, probably rocks, to increase the buffering capacity of the water, making the CO2 less effective at dropping the pH.
Have you checked tank decor with something acidic like vinegar to rule that out? If vinegar on a rock makes foam or bubbles, the rock shouldn't be in the tank - especially a CO2 injected tank. While you are at it, check some of your sand too.
Without a KH reading from the tank and the source water - we aren't totally shooting in the dark, but we're close to it..