Ok, PH 6.0, KH 0, gh 75 I hope that helps
The GH of 75ppm (= 4 dGH) is very soft water, and the zero KH means there is no pH buffering. So as the organics are broken down by bacteria which produces CO2, this in turn creates carbonic acid and the pH will lower. It will end up at some point where the chemistry/biology of the individual tank is stable. That explains what is naturally occurring, why the pH is and will remain low.
You have fish that will be fine with this, but you also have fish that cannot survive for long (the livebearers). Increasing the pH is not the answer, because the real issue for the fish is the GH. This is the level of dissolved calcium and magnesium in the water, and livebearers must have this so when the water is assimilated into the fish as it is continually the minerals are then extracted and used in the physiological process that maintain the fish's life. This means the GH must be significantly increased. The pH will naturally rise with it, and this too is important for these fish but the GH and pH must be addressed.
Soft water fish have the opposite issue. If they are kept in harder water, the minrals are extreacted from the water entering the fish through osmoregulation, a function of the kidneys. Unfortunately, these fish have evolved to live in soft to very soft water, and the calcium salts extracted from the water remain in the kidneys and block them. The fish weakens over time and dies.
I have separated this into hard water and soft water to make it easily understood. There is some degree of overlap, depending upon the fish species and the GH/pH. The livebearers cannot be maintained in good health unless the GH is increased to at least 12 dGH, and the pH will naturally be above 7 if this is done. This is quite an increase for the soft water species which are happy where the GH/pH now sits.
This brings us back to your initial question, of buffering with crushed coral. This is not the answer. First, crushed coral is not a good buffering because it lacks the magnesium. Dolomite and aragonite are good for this. Back in the 1990's I used dolomite in a nylon bag in the canister filter to raise pH because I was told it was "needed." About half a cup of dolomite raised the pH from below 5 to 6.5 in my 115g tank. Of course, the GH was still zero. Fortunately, I had soft water species in the tank so this low GH was ideal, and as the pH was stable (it never varied for several years from just the one half-cup of dolomite) and they adjusted to the pH (which was still below 7 fortunately) and all was well. But in hindsight it was unnecessary.
First thing is to decide on the fish. Increasing the GH/KH/pH means having to do this for every water change by preparing the water outside the aquarium before you can add it. It will be much easier to separate the fish, and either return the livebearers or set up a tank for them. If you do the latter, this means a tank with a substrate of aragonite sand, or using rift lake mineral salts. I did the substrate method when I did this for my mollies.
