I tested the pH of the water I left out overnight and it came to about 8/ |
Also tested tap water dechlorinated and that came out to be pH ~8.3.
Something about these numbers doesn't make to much sense.
This is making sense, I'll explain it.
First, the reason your tap water was testing 7.4 straight out of the tap and 8 after sitting is because of the dissolved CO2 in the water. Adding CO2 to water tends to lower the pH by producing carbonic acid. As the water sits, the CO2 dissipates out, thus "raising" the pH. Other factors also affect this in an aquarium, which I'll get to now.
When first set up, one can expect the pH of the tank water to be the same or very close to that of the tap water. And now we know both are around 8, that is normal.
In an aquarium, the natural biological processes tend to acidify the water. As this happens, minerals are also used, so the water slightly softens at the same time. The amount of mineral in the water may act as a buffer for a period of time, the length of time depending upon the hardness of the water. This is why we always ask about tap water hardness numbers, as that will give us a clue as to what we can expect in the way of a pH shift.
Adding calcareous substances to the aquarium--limestone, marble, dolomite, coral--in the form of gravel, sand or rocks made from any of these will increase the hardness. Adding wood, leaves, peat and similar organic materials will tend to acidify or lower the pH and hardness. Once again, the initial hardness of the water affects the extent to which this occurs.
Partial water changes will maintain stability because they introduce new water that counters the processes in the aquarium, maintaining a more even level of hardness and corresponding pH. If fish that prefer the hardness and pH range of your tap water are maintained in the aquarium, all should be good. It is possible to adjust the water's hardness and pH to suit specific fish that do better in different water, but this should only be done naturally, never with chemicals because the initial hardness buffering capacity will resist such changes, causing fluctuations which are far more stressful on fish. I won't go into all this now.