Hello Lorraine,
First, you shouldn't have used meds. Medications should always be the last thing you try to solve problems. Many medications are hard on the fish, and a lot of them can crush the already established cycle by killing off the good bacteria in your filter. If you must use meds, you could have used it in your backup tank.
PH dropped quickly because a lot of nitrate was formed in short amount of time. The production process of nitrate is acidic.
If you still haven't put your fish back into your main tank, you may do large partial water change as many times as you wish to bring the PH back. Acidic water isn't ideal condition for bacteria to multiply.
By the way, I also thought you have rushed it by getting fish too early. We always kept dosing ammonia for at least a few more days after both ammonia and nitrite are at 0ppm just to make sure the cycle is well established before adding any fish at all.
The next time if your fish is sick, you might as well bring the issue here to ask for help. We will try our best to give suggestions. A lot of problems can be solved by methods other than using medications.
We will try our best to give suggestions. A lot of problems can be solved by methods other than using medications.
