Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I think it is the sodium hydroxide that is raising the pH a bit.
In my tanks I have a high cationic exchange capacity substrate, and I think it also removes the sodium hydroxide, but I have no test.
I know it removes the carbonates.
The pH can go from the upper 7s (tap) to bottom of the test (6.0 or lower) and the KH drops from 4 or 5 dKH out of the tap to zero.
As noted much earlier in this thread:
Almost all aquarium plants are just fine in a very wide range of pH and mineral levels. There are very few that are so picky that you must give them really soft, acidic water. Do not go to that effort until you have a year or more of keeping planted tanks.
Almost all the hatchery raised fish will handle a wider range of mineral levels (GH is the most important) than their ancestors. So keeping them in water that is just a bit higher than the 'optimum' range is not going to be a problem for most fish. Certainly there are some fish that really do thrive only within a set range, and if your goal is to keep these species you will want to maintain the water that way.
If you want to, you could make a blend of your tap water and the bottled water, if your target parameters are somewhere in between the two.
Some water, confined to pipes, can be higher or lower in CO2 than water in equilibrium with the air. When such water is tested for pH right out of the tap it can be either high pH (holding less CO2) or low pH (Holding more CO2). When this water sits out overnight, or is aerated for half an hour or so the water will reach equilibrium with the air, gaining or losing CO2. Then the pH will change. If this is going on with your water it is a simple matter to age the water overnight before doing a water change.
If there is something else going on, such as the water company adding something like sodium hydroxide, you can figure out a way to remove it, or not. It is up to you. Many, many people keep fish and plants in water that is not optimum, but the stable conditions are best, rather than trying to alter something, but not doing it very well, creating variable conditions.
The worst of these is varying levels of TDS.
Fish regulate their cell water, salt and mineral content, and get used to doing it with water at a certain TDS level. When that level changes they need to alter their metabolism to maintain osmotic balance. This is not easy for them to do, and takes up to a month. Small changes in TDS, carried out over several weeks can acclimate the fish to significantly higher or lower levels much safer than a short drip acclimation.