Don't add antibiotics unless there is a known bacterial infection that hasn't responded to normal treatments. Mis-use and improper use of antibiotics has lead to drug resistant bacteria that kill people, animals, birds, fish and reptiles.
The fish does not show any signs of a bacterial infection.
The fish is skinny and this can be from intestinal worms, gill flukes or an internal protozoan infection. The fish could also have the Gourami Iridovirus.
Intestinal worms usually cause the fish to lose weight over a few months. The fish eat well, and they normally do a stringy white poop.
Internal protozoan infections cause fish to eat but not as much as normal. They lose weight rapidly (over a week or two). They do a stringy white poop.
Gill flukes suck the blood out of the fish and they slowly lose condition over months. Eventually the fish dies from low blood pressure and lack of blood.
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Are you sure the fish was not skinny when you got it?
Did the shop grab the wrong fish and give you a skinny one instead of a normal shaped one?
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Do a big water change, gravel clean the substrate, clean the filter, then feed the fish 3-5 times a day for a few weeks and see if it helps. If it does, then deworm the fish and treat them for gill flukes.
Isolating the fish won't help and could make problems worse, especially if the fish ends up in a bucket without a filter or heater. I would leave it where it is and try to feed it more. Then see how it goes over the next few days.
You should do more frequent water changes when feeding more often to keep the tank clean. A 75% water change and gravel cleaning the substrate every few days is good when feeding 3-5 times a day.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
