Let me also ask if you are able to "season" water and if you are doing partial water changes every week.

When you say you are regularly cleaning your filter, how thorough are you at that? The reason I ask that is that almost all us, especially when starting out with guppies and other fish, clean them out too well! :0

Some of the fish books actually suggest taking a plastic bucket (only used for fishy things since it was purchased - drat another expense!) and putting a little tank water in there. Sponge filters (among the very most efficient biological filters) are squeezed out only until they only "poof" a little dirt,

Likewise if you use filter floss (or the cheaper quilt bunting) for a box filter or power filter, knock out the big pieces and some of the dust, but return it to the filter while it is still a little discolored. Much the same can be done for those filter cartridges.

Understand that the cheap bone charcoal is about worthless as a chemical filter. Use it for drainage for house plants Activated carbon is active absorbing (actually "adsorbing" but ask a chemist what that means) waste materials and toxins for a week or two.

The charcoal can probably be left in there because it becomes a biological filter as good - guy bacteria grows on the charcoal. HOWEVER, I am told that if fresh water is added to the tank, the charcoal can RELEASE toxins back into the water (it is an equilibrium thing) so don't leave the charcoal there forever. So use it for houseplant drainage after a while too. ;)

The cheapest box filter "insert" is probably a handful of gravel where the gravel is about the diameter of your little fingernail. You can do a slosh and wash and return those babies to the tank real fast.

But if you are klutzy like I am, use the hose outside so your drain doesn't fill up with gravel. Sometime I will tell the story of the family who tried to FLUSH the FILTER FLOSS! The plumber had never had to deal with that before. (Oops, there's the story.)

A sponge filter from a well-established tank can be put under the laundry room faucet and squeezed repeatedly until clean or until your hand is tired. The little chlorine, which catches there, will dissipate in the tank. The bacteria in the tank will repopulate the sponge in a day or so.