Originally Posted by drhemlock2
i usedtropic marin KH alkalinity test and after reading the instructions it does not say ppm etc. i used the more sensitive test of 10ml x the number of drops shaking the bottle before hand and inverting the test tube a few times just to be sure. wait a minute it is ppm
OK, so 13ppm is less than 1 dKH, which is very low. As I said previously, this has no impact on fish or plants. But it does mean that your pH will likely lower naturally due to the biological processes. This also is not a problem, esp with soft water fish, but it is good to know it may occur so you can monitor it.
Assuming the pH of 6.6 is coming out of the tap, then weekly water changes of half the tank volume plus not overfeeding or overstocking will (should) not cause the pH to lower too much. I would check pH regularly, say every week just before you do a water change; keep a record so you can see if it is shifting and by how much if it is.
Did you ever determine the GH of the tap water from the municipal water people? I don't see this number mentioned earlier in the thread, and this is important for fish and plants. I think we can assume it is on the soft side, with the low KH and acidic pH, so that is not an issue for the fish but it might be for the plants if it is very soft. Can you get the GH number?