Thanks for this page. My current fish I've had for two years after a gap of some years - prior this I had a tank of fair goldfish which I kept for over twenty years without any sort of filtration equipment so this is all very new to me. Up until recently I ran a sponge type filtration in my tank which I cleaned weekly although when I bought it I was advised not to clean it. After a week it became obvious it needed washing out, it was foul! Since then I've swilled it on a weekly basis in discarded tank water and that seeems to keep it decent. However when I got my fish [I adopted him after his original keepers were being evicted] it came with another type of pump which incorporated a cartridge of charcoal/carbon which also had a removeable plastic grid which also needed a regular clean but it recently came to my attention that this filter had space for two of the cartridges which should be replaced monthly. I've been experimenting with this for the last month, changing alternate filter cartridges on alternate fortnights but to be honest, this seems to be less effective than the single cartridge was. The tank cloud far more frequently than it ever did before although it clears quickly and there seems to be far more detritus in the tank when I clean him out. What am I doing wrong?