Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
That does sound like a more extreme swing than is tolerated by the fish.
Here are some things I would try:
1) Call the water company and ask what they are adding to the water. See if there is something they recommend to neutralize it, or remove it from the water.
2) Run some water into a bucket and add a pump (water or air) and circulate the water for 24-48 hours.
3) Run some water into a bucket and add whatever you normally add (dechlor, plant fertilizer, other) and circulate that for 24-48 hours.
4) Find some filter media/insert that you can treat the water with. (see item 1).
If you have any of these on hand, then try adding some to the bucket of tap water you are aerating. Just test one material at a time. Of course you can set up several buckets with one material in each bucket, and leave one bucket with no additives. For this test I think it is OK if you just dump some of these into the bucket, or toss in a media bag of the material. If you get any positive result you can refine the process and force the water to flow through the material by using something like an Aquaclear filter The large open space for media would be filled with whichever of these does the trick.
Any of the 'pillows' sold for aquariums such as 'Phos-Zorb', 'Cupri-Zorb' and similar names. These products might also be incorporated into a mat of fiber that can be cut to fit in most filters.
... and here is the wierd one...
Oil Dri, Safe-T-Sorb, Turface, other montmorillonite clay. These materials have a high cationic exchange capacity, and MIGHT remove whatever the water company is adding. It is worth a try!
If you find something that seems to work, you would then need to test the water to be sure it has not removed something else. For example montmorillonite clay removes carbonates, so you would have to add back something like potassium bicarbonate to restore the KH.