Yuck, mold and gunk.

[safiredragon]

My tank has been going for about 7 months now. I do regular water changes, gravel sweeps, and of course change the filter cartridge regularly. My problem is that mold or something is growing on the parts of the filter that are above the water, but sometimes get wet, and around the rim of the aquarium, between the lid. Most of the stuff is green or white. Stuff is also building inside the filter components, like the intake tube and the place where the water comes out onto the cartridge. What's the best way to take care of this problem and possibly prevent it without hurting my fish? I have a 12-gallon tank that houses 4 long-finned danios and an otocinclus cat.

[mr gerbil]

Is the white growth occuring on the top of the filter, where there isn't any water? That's normalish.
Are you sure that the green isn't algae?
If it's a mould/algae/cotton like growth, it's something else...

[Cottontail]

I have the white "stuff" ontop of my filter. The green, is probably algae. Is it underwater? What you are seeing between the water line and the lid is probably because there was water to that level and you need to add some.