well today i was watching the guppies, and i noticed that a few were acting strangly. their tails and bodies are turing white.

ammonia-0
nitrite-0
nitrate-20
ph-7.4
no new fish
30 gallon tank, running 2 years
about 20 guppies, 4 mallies, and 3 corries.
wisper 30 filter
do about a 50% water change every 2 weeks

thanks in advanced

Does it look like a thin coating mainly over the body? It could be columnaris so google it for pics and see if its similar.

yeah i googled pics, and thats what it looked like.

How can i treat it?

What temperature is the water? columnaris likes high temperatures so lowering it is a good idea. Guppies can manage in cooler water so take it down to 18-20 celcius.
You will need to add salt at 2 teaspoons per gallon but we need to know if its just guppies in the tank first.
A bacterial med such as maracyn will be needed as well.

the water is 74 degrees (23C)
there are 4 mollies and 3 corries.
Do i need maracyn 1 or 2? I have maracyn 2, but i dont have very much of it.