OK, It's a water quality issue then.
Have a read of this topic which will explain what all the different levels are and explains why you need to test for them.
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you should definatley get yourelf a test kit as you need to test daily at the moment, just the petrol you'll use getting to the fish shop every day will add up to more than a test kit probably! £22 isn't terribly priced, you can get them cheaper, go down to about £15 in some places but there's always going to be variations.
If you can then find the API fresh water master test kit then that's your best option IMO. Alternativley the nutrafin mini master test kit is good. Basically though if you make sure it's a liquid test kit and it tests pH, ammonia, ntirite and nitrate then that should do.
the crucial thing for this fish is keeping the water quality spot on, so 25% water changes at least once a day, you can do twice a day if you like or bigger changes. this should minimise the ammonia and nitrite in the water which is what's causing him to become unwell.
the problem being that you've got poor water because you've not got enough good bacteria in the filter, the anti bacterial medication you're using is designed to kill bacteria. This means it can also wipe out the filter bacteria that you need.
Have you got something else you can put the fish in temporarily, just like a big mixing bowl or something like that? if so then when you are doing your water changes i'd take water from the tank and put it into the temporary 'tank', dose it up with medicine to the correct level, then put the poorly fish in there for an hr, add fresh water to the tank and then when the hours done put the poorly fish back in the tank.
that way he's getting exposure to the medication, water in the tank is kept as clean as possible and the filter bacteria aren't exposed to an anti-bac med so they have the strongest chance of flourishing as possible.
look through the yellow pages for fish shops in your area and call them all to try and find some myaxin though, you could also try contacting your local vet who may be able to prescribe something