It is diffacult to tell what exactly is wrong but I will make a couple of suggestions. I agree you should have a test kit. Do a 50% water change and vacume the substrate. Add 1 table spoon of aquarium salt per 10 gallons of water. Keep the nitrates at or below 20ppm as elavated levels of nitrates over extended periods of time cause low level stress and will compromise the fishes immune system and the fish will be less capable of fighting off infections, IPs etc. Always test the water befor a water change and let the numbers dictate the amount and frequency.
Treat the tank with Prazi or API general cure and follow the instructions on the package. Treat for the entire time as instructed it sometimes takes two courses of treatment to get it if the fish isn't eating. Have you noticed the fish passing white mucus type feces? I realize it's not eating but have you noticed anything? There is a posibilty it is an internal bacterial infection so if you see no improvement after treating for IPs we might have to switch meds and try another approch. As a side note Kanamycin (Seachem Kanaplex) is very well absorbed and is and excelent choice to use in conjunction with API GC if it's available to you. DO NOT USE JUST ANY ANTIBIOTIC it is important to use the right one the right way. If and when you need more info on this please post befor doing anything.