Sick Discus please help

[Dubs1281]

I had a discus who could not see well and was picked on by the rest of the group. I just thought he hung around the bottom because if those problems. He died two weeks ago and now one of my bigger ones has the same symptoms.
I took a look at his gills and they seem to be torn on the inside? Is this gill flukes?
My whole tank has had tons of fish die recently. Last night I did a pwc, cleaned the filters and replaced two out of the four cartridges because some disease might be on them.
What should I do?

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[Dubs1281]

Just put him in at and gave half dose on all meds except the erythromycin.
He's not laying on his side anymore and is exploring his new area.
When I transferred him he made a good attempt to evade the net and when I caught him he tried hard to jump out when he was out of water. Attached are pics off medicine I have added and him.

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[Dubs1281]

I got the plants today and decided to keep them in there to ease him a bit.

 

[Dubs1281]

He passed this morning. The water temp dropped to low.

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