Preventing mold on filter?

This is the second time this has happened, and I can't figure out the cause/how to prevent it. A white, fluffy mold/mildew grows on the filter cartridge just above the waterline. I have to keep changing the filter, since the mold will appear within a couple weeks of using it.

It is a 20gal tank with slightly acidic water that's been going for a little over a year. It has filter, heater, all the such, and is around 77 degrees. Whole heckton of little snails (idk what kind, "Gary the Snail" style brown shell and don't get bigger than a penny at best) and four Glofish, plus duckweed and some kind of lily pad-esque plant. I do a good clean of the gravel and a water change a couple times a month, since the ammonia/nitrite/nitrate levels are easy to keep at zero (I do test often). I treat the water with a water conditioner, Easybalance, and StressCoat+. I did cycle the tank, and this mold thing is a recent occurrence. I got the wrong filter cartridges, and they don't fit perfectly, but they do function. The fish themselves seem fine, all levels are stable.



The stuff just pops up out of nowhere and just got one of my succulents ill. What am I doing wrong?

PS: Just did a water test with a "5-in1 EasyStrips"
Nitrate: 0
Nitrite: 0
Water hardness is a little up there, but I can't give an exact number (this thing is hard to read)

 

Try to get a picture of it. You can't clean the stuff off? You throw out most of your cycle when you throw your cartridge out. Use it until it literally is falling apart. Try cutting a small slice in one side of the cartridge and dump the carbon out to get more flow. You don't need it anyway. I think you white stuff may be biofilm.

 

Try to get a picture of it. You can't clean the stuff off? You throw out most of your cycle when you throw your cartridge out. Use it until it literally is falling apart. Try cutting a small slice in one side of the cartridge and dump the carbon out to get more flow. You don't need it anyway. I think you white stuff may be biofilm.

Unfortunately, I didn't think to take a picture until AFTER I'd dumped the filter.
What's left got pretty squashed and deformed. It's usually a little rounder, fluffier, and whiter than this mess. I'll probably be getting the proper filter size tomorrow morning, and I guess I won't worry about the carbon filter.
I've tried cleaning it off, but it came right back the next morning.
Sorry about the terrible picture, the stuff is pretty messed up. Either way, thanks for the help!

 

That is so strange. What is the black part? Is that the carbon?

 

That is so strange. What is the black part? Is that the carbon?

The black part is where the water was. The cartridge was too big and stuck up over the waterline a bit inside the filter itself. The carbon part is on the backside. It's a little brownish in better lighting.

 

Do you swish it in tank water when you do your water changes? To clean it out some?

 

Do you swish it in tank water when you do your water changes? To clean it out some?

I was washing it out separately. It sorta disintegrated then came back the next day.

 

I may have found the issue, i.e. a disgusting source of white mildew near the tank.
I'll buy a new filter and see how things go

 

And?

 

IT'S BACK.
I didn't know whether to create a new forum or not, so I apologize. I caught it before it got to the filter cartridge and actually remembered to take a good picture this time:



I'm at my wits end, this thing is like white fluffy Ganondorf. Every time I think I've killed it it comes back.
Maybe now you can tell what it is, since the photo isn't of a muddy crushed husk? Will it hurt my fish?