But sounds like u have had a pH drop. Let's assume (test before u start) ur tap is over 6.7. ur bacteria have become inefficient. reduce feeding do tiny water changes daily. Like 5-10% max! If possible get a slow release buffer. Tiny pieces of lime or marble gravel talking tiny(need tank size and temp) ur want to very gradually add these. A small bag of crushed coral will work as well. Before u start ur want to also establish ur kh. Ur use the buffer to add a slow release carbonate source to react with the acid lowering ur pH. That coupled with water changes should allow u to bring up ur pH to at least the mid 6.
Now some fish are fine with 6 but u gotta have more bacteria so more media and more oxygen.
Easiest bet is raise ur pH to ur tap water level with very small amounts of buffer and water changes.
Now if u want instant results can use sodium bicarbonate but it'll be very fast and Burn out quickly and drop back down to where it is now.