Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I have water similar to yours.
GH and KH out of the tap are usually 4 German degrees, though it can vary a bit.
pH is usually the upper 7s and often enough is into the 8s. One aquarium keeping friend measured it at 9.0 a couple of summers ago.
The local water company adds sodium hydroxide. There are other things that your water company could be adding. Call them an ask. That is how I found out about my tap water.
If you can remove whatever they are adding, then the pH ought to come down.
Try these ideas:
Filter some tap water through some commonly available things and see if it helps. The easiest way is to put some water in a 5 gallon bucket with a bubbler or small fountain pump and some of any one of these materials. Test at the start, then about every 12 hours. If there is no change in 24 hours, then the material is not doing anything to help you.
Peat Moss: Garden store stuff, but watch out for fertilizers or surfactants. Put a generous handful in a nylon stocking.
Any of the *sorb sort of filter media, such as cupri-sorb, phos-sorb and so on. While each one seems quite specific, in fact each one may adsorb other materials, too. (These are pretty expensive if you have to start using them to prepare the water for every water change)
Any of the Oil-Dry sorts of materials. These products have very high cationic exchange capacity, but do not adsorb all minerals. They might help with this problem, though. I have many tanks with Soil Master Select, also some Turface and Safe-T-Sorb.
All these tanks show this response: The substrate removes the KH (per tests) and allows the pH to drop (per tests). But I think that if the sodium hydroxide was still in the water the pH would not drop so much. I think (but have no way of testing) that the substrate is also removing the sodium hydroxide. Maybe it will remove whatever the water company is adding to your water? These products do not alter the GH, so do not adsorb Ca or Mg. Put them in a nylon stocking, too. I have no idea how much will last how long, though. Perhaps a cup or so in 4-5 gallons of water?