There are two mulms – “bad” one, and the “good” one.

If we have less surface area (and too much flow!) than needed for a given bioload HMF will clog and turn into plane sponge filter requiring often cleaning.
We will have too much “bad” mulm in it (almost not biodegraded, high lability).
The same as we have in a canister filter as nitrification byproduct - detritus.
As canister is not good place for heterotrophs, it accumulates quickly.
Too long without cleaning - and it will turn reductive.
In a HMF made good a lot of “good” mulm – almost totally biodegraded, low lability mulm (micronized). It has to stay there as long as possible.

As I say HMF have very active low labile mulm which works even better than mulm in a gravel.
Making good HMF you virtually getting one more substrate surface aria of you tank in filter (!), and this mulm in HMF works just like the most valuable part of it – top thing layer of mulm laying on substrate.
Even more, mulm in HMF is much more active – more flow, more oxygen, more organic food for “critters”.
It works just like an UGF but without those disaster after several months.

To do not clog HMF with large debris we need good prefilter. Than cleaning HMF virtually needn't.

Composting trimmings with Earthworms is great for Biodynamics experiment, but I can’t see any sense in it for a common tank as we have Earthworm castings.
We can make ferty steaks from this compost the same way as we do from earthworm castings – freezing it in 2ml syringes.
In 90-s some guy made an experiment with trimmings (see TheKrib).
As I remember he did not used proper mineralization methods or composting with eartworms.


naman