Originally Posted by Calmwaters
No problem. I did not do the water change because I figured that would lower the Ph even more which I don't think would be good for the fish.
Amanda, if your tap water is pH 7, doing a partial water change would raise the tank's pH slightly (from 6 to maybe 6.2 or 6.4) which is not a problem; mine do that every week. However, the real issue is finding the reason the tank is 6.
MoneyMitch is on the right track with the hardness issue, but I'm suspecting the opposite is the case--your well water is probably very soft and thus has no buffering capability. My tap water is like that; GH and KH are 0 or at most 1 ppm, and pH is 6.8.
When an aquarium has been running for a few months, as you indicate yours has been, the natural biological processes will cause the water to slowly acidify and the ph will drop. If the water has a degree of carbonate hardness (dKH) that carbonate hardness will act as a buffer and the pH will not lower much if at all. At some point though it may, when the buffering capability is reached, and then it can crash (the pH). However, depending upon how high the KH is, regular partial water changes will normally keep the tank pH stable because they replace a percentage of the water with fresh water having the specific KH.
If your water has very little or no KH--and well water could be basically rain water (which is always acidic) in some areas, as opposed to water from springs running over mineral beds--then the ph will slowly drop as the tank matures. The weekly partial water changes will help to keep the pH up and slightly stable, but as there is no buffering KH this is risky.
My solution in such a case is to place a small amount of a calcareous material in the filter (or tank), such as dolomite, crushed coral, marble chips; limestone rock also works but much slower. I use canister filters, so putting a nylon bag (made for filter material, you can get these in fish stores) with some dolomite gravel (you can buy this from fish stores, people with marine tanks often use dolomite gravel) in one of the filter baskets with the media works best; you can also use crushed coral gravel, or marble chips. These materials slowly leech calcium into the water stream as it flows over them, slightly raising the hardness of the water and thus the pH. It doesn't take very much, I have about half a cup in my filters on each tank and it adds about 2 ppm KH to the water that is 0 KH out of the tap. In 12 years, this has kept the pH in my tanks from dropping below 6. The dolomite needs replacing maybe once a year.
You could get an API test kit for hardness (that includes both GH and KH, they also have them separately but both KH and GH are good to know) or take a sample of your tap water to the fish store and ask them to test the KH and GH. KH is the more important for the buffering capability, but the GH is useful to know. Once you know this, you will know what action if any to take.
Before speculating on other possible causes, I would suggest testing the water as this is usually the answer. In the meantime, continue the normal weekly pwc, 30-40% max, as letting it go without a pwc will probably allow the pH to continue lowering. On no account attempt to adjust the ph until you know the hardness; fluctuating pH greater than .4 can seriously affect some fish; a stable pH, whatever it is, is preferable. There is a normal diurnal fluctuation of .3 to .5 in all planted aquaria between daylight (pH rises during photosynthesis) and night (pH falls) and the fish have no problems. But sudden changes in short periods can be fatal.