Clownfish Seizure and death after freshwater dip. Help :(

So after my other clown (true percula) died suddenly (possible brooklynella or velvet) I got 2 ocellaris clowns. At the store they looked fine, 2 hours in my tank, they're now coated in slime. All my other fishes are fine. I've medicated my tank with inch attack because that's what I thought it was.

So afterwards I notice the clowns were suffering pretty badly, swimming near sand and breathing heavily. Here I decided to give myself more time to get medicine tomorrow so I did a freshwater dip. I used the same bucket of water for both of them. The first one I put in was swimming disoriented for the entire time (5 minutes) the second clown was more oriented and stayed still the entire time.

After I put them both back in the tank, the first one is fine and swimming around and the second was swimming vertically for a while and then went crazy. Swimming and hitting things like it was having a seizure. Then the sir curation pump took him around the tank. I quickly netted him so he would hurt himself. After a good 2 minutes of going crazy in the net, he sudddled to the bottom and stopped breathing.

What did I do wrong, water matched before the dip. Had less slime then the other clown but still died. I did a 20 percent water change after the dip.