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My shark catfish has been spending lots of time at the top of the waterline, like he is eating air, which I think he is. This morning he has suddenly got a bloated belly and he looks awful. Because of his strange behavior at the top of the tank yesterday, I checked the water before I went to bed. Ammonia 0, nitrites 0, nitrates 20. Salinity is 1.004.

I have recently raised salt from 2tbs in 5 gallons of prep water to 3tbs, but it was a gradual increase since I am only adding salt to change water. This shark cat is 2 3/4 inches long.

I have tried feeding him freeze-dried bloodworms for the first time this week. He is a bottom feeder-has never eaten anything from the top before. I left a few pieces of the blood worm floating around the edges because it takes him awhile to find things. I THOUGHT he was finally going up and eating some of the bloodworm. It was all gone yesterday morning. He has spent more and more time up there, almost like he is sifting the water for more blood worms. But now his belly is large, he cannot more around very well.
It is like he has filled himself up with air.

Tried feeding him one of his thawed frozen shrimp last night and wouldn't eat it. Immediately did a 30% water change this morning when I saw him, as a precaution.

I am REALLY worried about him! He is not swimming much at all, and is nearly motionless when he hangs at the waterline. Any ideas???

Additionally, he is definitely gasping, with his barbells right at the water line. The tank is well aerated. Plus I just did the 30% water change (water had stress coat added and had been sitting for about 20 hours).

I have recently raised salt from 2tbs in 5 gallons of prep water to 3tbs, but it was a gradual increase since I am only adding salt to change

Question! What type of salt are you using?
& What is the GH & KH values