That should be before the change. I asked him to keep some of the water he was pulling out
But I agree, I think it's probably the parameters in the tank stressing the fish, since they've been used to a cycled tank. I'd do a full dose of
Prime for the tank to keep them safe overnight, and see how things are looking in the morning. I'd also redose the Stability for the day since you've done another big water change.
I'm beginning to think I'm too sleepy, since I completely just contradicted myself within a few minutes there...
nitrites are a concern, but iirc you've been dosing the tank with prime for the full tank, so they should be alright with the .25, even though it's not ideal.
Edit: So, here is what I would suggest. Add your Prime and Stability, check your numbers in the water when you get up tomorrow, if
ammonia and nitrites are still 0-.25 and numbers look like they are under what Prime covers and the fish are still acting off, pull apart the filter and clean it all...