Omg! I have a hith prob, what do i do?

ok guys i have treated my fish with prazi and metronidazole and it seems like they cant handle the meds. I do a 25% water change before i treat them and then wait like and hour or two and come back and they act like they cant breathe! this is scaring the crap out of me. i dont know if im just freaking out or if im doing the right thing by doing a 30% water change b/c i feel like they are going to die if i dont. im torn. i know when i change the water after medicating that i am diluting the medication. i dont believe there's a lack of O2 b/c i made sure to add an extra filter and an air pump to the tank before i started treating them along with my 2 penguin 330's. so that makes 3 hob filters all rated for 60 gals and up and then a canister filter ( which i turned off) and then the air pump. what am i doin wrong?