my cat was damaged, and aquaculture fish doesn't heal them only normal fish

What version of Aquaculture are you using?

1.16.1-2.0.18

Testing with Aquaculture 2.0.18 for 1.16.1 I'm unable to reproduce the problem

My best guess is that the fish you tried with, is not actually from Aquaculture 2.

the thing is, they eat it but their health doesn't go up

I didn't actually check that, I'll check it again tomorrow.