It's definitely getting worse. You need to treat the tank with something asap.
Antibiotics require a vet prescription in a lot of countries due to drug resistance caused by improper use and mis-use over the decades, as mentioned by Essjay. We have the same thing in Australia. The Waterlife Myxazin is the only thing I can think of that should be available where you are and that might help.
Doxycycline is a tetracycline based antibiotic that wipes out filter bacteria and can be used to treat fish with known bacterial infections. But you will need a vet prescription for it. You probably won't get it through customs if you try to import it unless it's labelled as something else. I doubt the fish will survive a month so look for something local.
If you need to use antibiotics they work best in bare glass tanks. Your tank is pretty close to that so just wipe the inside down with a clean sponge, clean the filter and do a big (90-100%) water change before treating and re-treating the aquarium. Then monitor ammonia and nitrite levels for a few weeks after because some can wipe out the filter bacteria. You can use antibiotics with salt in the water is need be.
You should increase aeration when treating fish and remove carbon if using liquid or chemical medications. You don't need to remove carbon when using salt but you will if using Myxazin or anything else.
If you can get antibiotic food that should help but you need to start feeding it asap and feed 3 times a day (8 hours apart) for 1-2 weeks. Feed them as much as they can eat in a couple of minutes when you feed them. Check the instructions on the packaging just in case they differ.
A combination of medicated food and a medication in the water will hopefully help.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
If you have big rocks or driftwood in the tank, remove these before measuring the height of the water level so you get a more accurate water volume.
You can use a permanent marker to draw a line on the tank at the water level and put down how many litres are in the tank at that level.
There is a calculator/ converter in the "FishForum.net Calculator" under "Useful Links" at the top right of this page that will let you convert litres to gallons if you need it.