7 inch tig in a 20g with not much current ?
Recipe for disaster .... should in a a bare minimum of 75g with massive current.
Why did you treat the fish with Metro in the first place ? What dosage did you use?
Metro is a last resort drug only ....it can cause more issues then it helps in catfish.
You state the water is fine? That doesn't mean much to me ....what is fine ?
What are the actual test results ?
PH, GH, KH, Ammonia , Nitrite, Nitrate, Temp
You don't have the test results? Test the water and find out ...
What filtration ?
Without detailed information I can't help you.