08-26-2010, 12:41 AM
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MoneyMitch is correct, something in the tank (or possibly some media substance in the filter?) is leeching calcium into the water, and this raises hardness and pH. All else being equal, a tank will gradually lower in pH as the water acidifies, depending upon the KH of your source water (tap water). The higher the KH the more it buffers pH preventing fluctuations. With a pH of 6.4 in the tap water I would expect the hardness (GH and KH) to be very low, though it sometimes can be different.
That said, to explain why, now to what if anything you should do. Livebearers (molly and platy) need harder basic (alkaline) water. The 7.6 is [perfect, and as mentioned above, this is probably being caused by something calcareous so the hardness of the water will be higher than out of the tap too. Fine for livebearers. But not for rummys.
Rummynose tetra should never be housed with livebearers because their preferences in water are too different. There are three species of rummys, the most common now is Hemigrammus bleheri. If you check our profile of this fish (click on the shaded name in posts, or use the second tab from the left in the blue bar across the top) you will see it recommends a pH below 7 and preferably below 6.5. The habitat water pH as stated there is 3.7 to 5, and I have this from Heiko Bleher who has explored the Amazon region and discovered hundreds of fish including this beautiful species as it mentions in the profile. And all fish are wild caught, so maintaining this fish in basic hard water will be detrimental over time. The result will, at best, be a shortened lifespan, and could be worse. Calcium deposits build up in soft water fish and block kidneys, all sorts of problems. Water parameters are very important for wild caught fish.
If it were me, I would decide on a soft acidic water fish tank or a basic harder water fish tank, and proceed accordingly with suitable fish, re-homing whichever to another tank or the store or another hobbyist.