Are you sure it's fungus?
What sort of fish is infected?
Can you post pictures of the sick fish?
Harmful fungus in fish normally gets into open wounds. If the fish has not been injured, it might not be fungus.
Fungus (saprolegnia) in fish can be treated with salt (sodium chloride), Methylene Blue, Triple Sulpha and a number of other medications. However, salt is the cheapest and safest. Use 1-2 heaped tablespoons of rock salt per 20 litres of tank water. Leave salt in tank for 2 weeks. the salt will not affect fish, plants, snails, shrimp or filter bacteria. The fungus usually goes after a few days but you leave the salt in there for longer so the wound can heal up properly.