Boraras brigittae ? Mosquito Rasbora (Rasbora urophthalma brigittae) ? Seriously Fish
Note that these fish accept a wide range of GH (from 1 to 10 German degrees of hardness), but prefer the pH to stay on the acidic side of neutral.
pH is not a stand alone value.
The minerals and salts in the tank control the pH.
You control the pH by controlling the things that control the pH.
Test and post back:
GH, KH, TDS, pH of tap water, and repeat the pH test:
Test it right out of the tap, then set some water aside, exposed to the air and test again at 24 hours and 48 hours.
GH, KH, TDS of the tank water.
Next, run a few experiments.
1) Often the pH is high because of something in the tank such as limestone, coral or shells that are dissolving, adding calcium carbonates and magnesium carbonates to the water. Test whatever is in the tank by placing it in a separate jar of water (RO water, tap water... whatever) and testing GH, KH, pH, TDS the next day, then every few days for perhaps a week. Test a handful of substrate, a chunk of rock, the ceramic merperson...
2) If the tap water KH and GH are contributing to the problem you will probably end up blending reverse osmosis water with the tap water to lower the KH for the pH, and lower the GH for the fish. You can get started testing this idea with a gallon of RO water from the store.
Make several blends, perhaps a couple of cups of each:
25% RO + 75% tap
75% RO + 25% tap.
Test GH, KH, TDS, pH on all these, and use the one that comes closest to what the fish want.
Post back with all these results.
To implement changes in the water chemistry, go slow. The fish have had time to adjust to whatever is going on now, and you need to alter the conditions slowly to get it back to optimum conditions.
Make up the water the way you will want the tank to end up.
The first week do 2 water changes of 10%. If the tank needs larger water changes, perhaps do to rising nitrates, then make up the refill water to match the current conditions (GH, KH, TDS) in the tank.
The 2nd week do 2 water changes of 25%.
The 3rd week do 2 water changes of 33%.
The 4th week and going forward do 2 water changes of 50% until the water parameters are where you want them.
Then you can stop doing this many large water changes, and monitor the tank. See how long it takes to get just a little bit out of the correct range, and how much of a water change you are willing to do. This will set your schedule to keep this tank in the right range.