Sounds like too much stress at once between the large water change, chemicals, temp swing, and new fish. You say you have a lot of fish is this 75 gal. What are they and how many? How much water do you usually change and how often?
I'm nothing close to an expert,
But I've learned to stay away from chemicals unless ABSOLUTELY necessary.
When we started keeping fish we killed quite a few trying to use chemicals and water altering meds.
Hope the wc helped!
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Thanks a bunch guys, everything is back to normal. It wasn't the chemicals, it was time to service the canisters. Only fairly recently have I tried keeping live plants. I shoulda realized it wasn't going to take very long fill my filters and it was obvious when I opened them up.
Pretty rookie, I know. I appreciate all your guys help, Thanks again