One of my goldfish has been staying at the bottom of the tank and looking like he's struggling to breath. He's been like this for a few weeks now and I feel so bad. I have 2 comet goldfish in a 30g. At first I thought it was a nitrite problem, so I tested the water and it turned out to be pretty high. I ended up buying more plants and changing out water and now the nitrite problem is resolved. But my one fish is still the same. There's no redness on the fins or belly. No difference in appearance. He's always been kind of fat because he eats a lot more than the other goldfish. That is why I'm guessing it's constipation. He shoots up to the top every once in a while like he needs more oxygen.The other fish is perfectly fine. I've stopped feeding both and started giving them peas. I think it's helping a little bit. My question is, can I use Epsom salt in the tank while my other goldfish is in it too? I don't have another tank to separate them so that is why I'm asking.