Is the fish eating normally or at all?
What does its poop look like?
When fish suddenly bloat up, breath heavily and stop eating, they have a major internal problem (usually organ failure caused by bacteria, quite often TB). There's no cure if it has organ failure and has stopped eating and the fish usually dies within 24-48 hours of showing these symptoms. Euthanasia is the normal recommendation if it has stopped eating.
If it's still eating and doing normal coloured poop, then an Epsom salts bath might help or it could just be full of eggs.
The heavy breathing is usually associated with major internal pain and usually indicates bad things (death is on its way).
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Epsom salts bath can be done several times a day. I'm not `100% on dose rates but think it's 1 heaped tablespoon of magnesium sulphate for 20 litres of water. You put the fish in it for 15-30 minutes then put it back in the tank.
Use tank water for the Epsom salts bath. Have an air stone bubbling away in it.
Care must be taken when catching or lifting any fish out of water especially if it has an internal problem. The weight of the body lifted out of water can cause damaged organs in the body to get squashed and damaged even more.
If you have to move a sick fish out of the tank, carefully catch it in a net but keep it in water. Put a container in the water under the fish and lift the fish and net up in the container. Then put the container into the bucket with Epsom salts and let the fish swim out.
Epsom salts normally needs to be dissolved in hot water. Do this in a small container of water and add that liquid to the bigger bucket for treatment. Make sure the water in the treatment bucket isn't too hot for the fish.
