Hi and welcome to the forum
check your water quality and do a 75% water change and gravel clean the substrate.
clean your filter if it hasn't been done in the last 2 weeks. wash filter materials in a bucket of tank water.
pimafix and melafix are not worth using and can leave an oily film on the surface that can cause fish to struggle to breath. If there is good aeration/ surface turbulence its less of an issue.
Assuming the water is good and the substrate and filter are clean, and the temperature is correct, get a broad spectrum fish medication that treats fungus, bacteria and protozoan infections and treat the tank.
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.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will adsorb the medication and stop it working.
Wipe the inside of the glass down with a clean fish sponge. Do a 75% water change and gravel clean the substrate. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.