If its white spot, or ich as its also known, yes it can and WILL to your other fish. I have an outbreak of it in my tank at the minute and i am treating it with Interpet No6. I have administered one dose so far and the next dose is due tomrrow and there are signs on improvement already. I just cranked the temp up to 28 degress(depends on what fish you have and their temperature range) as ideally you want to heat it up to get the spots to their free swimming stage where the meds will kill em! You cant really do anything while the spots are on the fish, only when they detach. If left untreated it can cause the fish to struggle to breathe through thier gills and be fatal. I am no expert, but i just saw a few white spots on one of my fish's tail fins and within a day it had spread. Hope that helps. GRJ
Its called Interpet No6 and should be available at most lfs's. I got mine from Pets At Home and it was £3.99. Its easy to use and come with all the instructions you need to work out the dosage. HTH GRJ
Oh god yes! I orgot about that! I am new to all this myself. I dont have any carbon in my filter anyway, just have it there to remove meds after treatment! Cheers for reminding me about that though!GRJ