i have a jewel tank.not the same size but went through changing all the sponges . the only one i change now is the white one weekly but bought a big bag of filter floss instead.same thing but in large bag or box. much cheaper than jewel ones and does exact same job. dont use black sponge anymore...
meant to add only cost me £4.95 i think he let me have it cheap as its about 10'' long and 8'' high and i was dithering whether to buy any more stuff ;)
dont understand why some float some sink.is it just because of it being hollow.ive bought 3 bits lately including this huge bit soaked for few hrs and straight in tank, sunk straight away and hardly leaches. maybe pre soaked? but didnt looklike it in shop.