If you have to use anti-biotics, use them in a clean bare container with no gravel, no wood or porous ornaments, and no filter. Just have a heater and airstone bubbling away, and maybe some plastic plants. Most anti-biotics will wipe out filter bacteria as well as bacteria affecting the fish.
Before you re-treat the tank, you should wipe down the inside, do a 100% water change with dechlorinated water, and then treat the tank.
If you treat the main tank, the anti-biotics will get wasted on bacteria in the gravel, filter, on the glass and plants. This is why you use a bare container for treating them, so the medication is used to treat the fish and not wasted killing other bacteria in the tank.
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Before you treat the tank, do the following things.
Work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
If you have big rocks or driftwood in the tank, remove these so you get a more accurate water volume.
When you measure the height, measure from the top of the substrate to the top of the water level.
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 bottom of this page that will let you convert litres to gallons if you need it.
If you can't get a separate container to treat the fish, remove carbon from filters before treating the main tank, otherwise the carbon will remove the medication.