depending on what is in the anti-fungal remedy, it might not make any difference to the problem. If the fish have a bacterial infection, which is more likely, you will need a anti-bacterial treatment, not anti-fungal.
Can you describe the symptoms that the sick fish have?
Perhaps do a big (50-75%) water change and complete gravel clean. Check the filter and clean that if it hasn't been done in the last month. Then try a broad-spectrum medication like "Waterlife Protozin", "Wardley's Promethyasul", or Triple/ Tri Sulpha.
To work out the volume of water in the tank
measure Length x Width x Height in cm
divide by 1000
equals volume in litres
When measuring the height, measure from the top of the gravel to the top of the water level. If you have big rocks or driftwood in the tank, remove them before measuring the height.
Remove carbon from the filter before treating otherwise it will absorb the medication out of the water.