You don't have to worry about whitespot except when introducing new fish usually. This is the primary place that it comes from. The fish are shipped in large numbers, in small containers, and so one little patch of whitespot can quickly spread... so dosing every 3 months to prevent it is pointless unless you are getting new fish and swapping fish every month or so. Once you have an established tank with no new editions and everything is healthy for a month or so, then you shouldn't have an outbreak of whitespot as there is no means of it to get into the tank... you would just be wasting time and money and possibly the health of the fish as some scaleless species can become seriously ill from alot of the medicines for whitespot