K first of all NEVER run the foam under the tap, the beneficial bacteria in the tank lives in the gravel and most of all the foam, if you rinse it in tap water you'll kill it all off and the tank will have to be cycled again, always use tank water for this. What type of filter have you got, my fluval I tend to rinse out 1 foam ever month and alternate it, the ehiem canister I leave for about a month 2. Some people do more often others less. If theres more than one foam in the filter its probably best to alternate them.