I'm about to rip you a new one because you used anti-biotics to treat a fish that DOES NOT HAVE A BACTERIAL INFECTION. However, that should not be taken as to the behaviour of most people on this forum. Most of the people here are nice but I am sick and tired of people using anti-biotics when the fish do not have a bacterial infection.
Why do people persist in using anti-biotics when they don't know what the problem is?
Just because anti-biotics are available, it DOES NOT MEAN YOU HAVE TO USE THEM FOR EVERY LITTLE THING.
Do you take anti-biotics when you scratch yourself?
Do you take anti-biotics when you have lice or nits?
Do you take anti-biotics if you have a runny nose?
The answer should be no.
Anti-biotics should only be used as a last resort for know bacterial infections that have not responded to normal treatments. Improper use and mis-use of anti-biotics has lead to drug resistant bacteria that kill people, animals, birds, reptiles and fish.
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Now that is out of my system, your fish is covered in excess mucous that is caused by something in the water irritating the fish. It could be poor water quality, chemicals (including anti-biotics), or an external protozoan infection (costia, chilodonella or trichodina).
Test the tank water for ammonia, nitrite, nitrate and pH.
Do a 75% water change and gravel clean the substrate every day for a week or until the problem is resolved.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If you want to add something, add some salt, (see directions below).
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SALT
You can add rock salt (often sold as aquarium salt) or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate (4 heaped tablespoons per 20 litres) will affect some plants and some snails. The lower dose rate (1-2 heaped tablespoons per 20 litres) will not affect fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.
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And welcome to the forum
And there's no point moving the fish out of the main tank because whatever it has, is in the main tank.