Thor has torn fins. I know its not fin rot because no white edges. I think its nitrate poisoning and I ve done a 50% water change. I ve moved him to a quarintine tank and added some salt.
Yesterday he got stuck in the filter suction in the big tank, but he wasn't injured. Yesterday he was acting tired and lethargic.
Today he is more active but he still has torn fins.

What else should I do?

You already quarantined him, did you add a cycled filter? That's the most important part when treating a stressed fish
(Torn fins are common signs of stress or bad water quality)

Try feeding him extra nutritious foods, like vegetables and live foods, and monitor the water quality daily, change out a little bit every now and then if the tests tell you to. That's really all there is to do about it.
I bet he'll be alright! Keep us posted, and good luck!

And then bad he got stuck in the filter again. Can barely swim. I'm not sure if he'll be okay much longer. I'm not sure if to add salt agian.

I have never in my life had a fish get stuck in the filter intake... is there a way you could post a picture of this filter?
It might be an idea to, like Skwishee said, cover it a bit with some ornament a bit away, or you could exchange it completely to something safer.

Are you sure your water quality is good? Is he getting enough food? A healthy fish shouldn't go get dragged into a filter like that

That's so horrible
Nitrate and nitrite poisoning isn't to toy with as it turns out for both of us...

Thanks for all your help. I geuss I treated too late.

This is almost the same way Bucky died.