Sponge filters, like any bio filter are nitrate factories. That is their function. Keeping them clean will reduce or low the rate of nitrate build up. They do require vigorous squeezing to get then clean. The nitrifying bacteria is well adhered to the sponge and not easily removed. Every tank is different as to how often they "need" to be cleaned. Getting the solids out so they can function properly is a good idea, for any filter.