I have never noticed a significant impact on pH when using Prazi. Albeit I don't use it in a display tank with an apex that has true live monitoring.
How is the oxygenation in your tank? Is the tang one of the larger fish in your tank? Prazi depletes oxygen very quickly and a fish that isn't active or hiding during a prazi treatment can indicate that the oxygen level in the water is low. So they aren't active in order to preserve oxygen. Crank you powerheads upward and get some really heavy surface tension going. Make it look like a river. See if that helps.
If you had a large bristleworm population or any other ornamental worms then the die off from dosing praziquantel could cause die off and an ammonia spike. If you have registering ammonia I would highly recommend a water change ASAP as well!