Filter experiment
 

I have a small, 10 gal tank. I like the aqueon quiet flow HOB filters, but unfortunately they use cartridges. Whenever you change out a cartridge, you loose most of the nitrifying bacteria and have to go through another cycling phase.

So solve this problem I added cut up filter foam. Now hopefully I can change my cartridge without loosing all my bacteria. What I would like to do in the future though is to find foam big enough to replace the cartridge entirely! They are such a waste of money. I try to wash them off in my dirty water after water changes, but they eventually break down and get cruddy.

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I added foam at the beginning and end of the filter.

At one time all I used was hang on back filters, thought they were great. But after going through what you are experiencing all of my tanks under 55 gallons have inexpensive sponge filters in them now. They last for years and when they need cleaning rinse them in the bucket of the used exchaned water during the water change. This way your bacteria won't be killed. I over size the filter by one size. ATI is agood one and if I remember right the hydro V was around $20.00 dollars. Another advantage sponge filters don't suck up the fry.

What you've done should work well. The pre-cartridge foam will also lengthen the life of the cartridge and the post cartridge foam will only rarely even need rinsing.
Sponge is an excellent platform for beneficial bacteria.

I have an AqquaClear 70 that's just using sponges and fiber, effectively making it a sponge/HOB.

Sponge filters are good fro heavily planted, small tanks. No doubt about that.
This came as a kit though at a great price. If I had went with something else, even a small sponge filter, it would have cost more.

Anyone know of any large sponges that I could cut and fit into my filter to replace the cartridge entirely?

Isn't that extra blue surface area designed to keep the nitrifying bacteria?

Yes, but it's not nearly enough in my opinion.

I use this:
But get it at Pet Smart. Lasts a few months before needing replacement, not as long lasting as a sponge, but is finer so good for a later stage polishing.

I also use this on the intake of my HOBs:

Fluval Edge Pre-Filter Sponge - Fish - Sale - PetSmart

Works on any HOB (mine are the cheapie ones that come with tanks) but they last forever and keep the big gunk out.

My tanks are all fully planted though, so I personally care less about the bacteria (I don't really need it).

Instead of sponges for the Aquaclear 30, you might use sponges for the Aquaclear 70 and trim to fit. HOWEVER, I think the (perhaps only) beauty of the cartridge filter is that the cartridge is easily pulled to rinse off the gunk and put back in on the fly.
Just my $.02, but I'd continue to use the cartridge as a pre-filter to the sponge bio-filter platform.