Most pH test kits include pH up and pH down. Keep the pH at 7.0.
pH up and pH down are just a work of the devil. They can cause more harm than good by swinging the ph without your control. Bin them if you obtain them.
pH doesn't bother the fish but KH and GH will.
Try testing the following:
GH, KH, pH, ammonia, nitrites and nitrates
Maintain ammonia and nitrites to 0. Ammonia is more toxic at a higher pH.
Nitrates should be 10-20 ppm. Max is 50.
To increase: the use of potassium nitrates.
To reduce: More water changes(make sure tapwater does not have nitrates) or the use of fast-growing plants like elodea densa and ceratophyllum demersum.
Lower than 10 increases the chances of BGA(blue-green algae). Blackout can eliminate this.
Goldfish=Higher nitrate levels due to poop.