Guppy Fungal Infection

[Crossfire]

Well, I just came back from PetSmart with 6 guppies....and now I have to go exchange one of them because it had a massive fungal infection!!!! They better exchange him for another But now I'm concerned that it could have spread to the other fish in the tank...I can't use medicine because it will ruin my filter, as I've read...any other methods i could use to ensure that it doesnt spread?

 

[soybean]

A larger than normal water change.

No need to medicate unless you see fungus on other fish. Anti fungal meds won't hurt your filter but you would need to remove any carbon from the filter first, as this removes meds and chemicals from the water.