Help, my bristle catfish is lying there very strangely?

Hey guys, I have a 180L tank with 7 Kangal fish, 7 Mollies, 3 gouramis, and 5 Bristlecone catfish (+baby catfish, unfortunately I can't say how many). And now the problem. I got the old catfish from a good friend because he no longer had the time or desire. Unfortunately, I don't know how old the catfish is; I'd say around 8-10 years. The catfish has been in my tank for about 2.5 months and has actually settled in well. But since yesterday, it's just been lying there and doing nothing. Today I simply reached in and touched it. I could pick it up and do basically anything with it, and it didn't care. Today, it lay on its back for a short time. I know they sleep like that sometimes, but it didn't look healthy, and it was also breathing heavily. Today, I gave it just over 30L of water, as is the case every week.

Now my question is whether and how I can help him. Are there perhaps too many fish? And should I put him in a different tank (one with neons, a betta, lots of shrimp, and a baby catfish)? Please help me.