Mulm naturally accumulates at most substrate surfaces of every aquatic ecosystem on this planet and is broken down within top 1.5-2” oxic layers of substrate as long as you make sure substrate is well circulated. If you don’t have adequate circulation then you start having problems with stuff like sulfide etc that nature deals with in anoxic layers of the ecosystem which is always well away from top layers.
This little tank was dry hardscaped, flushed through and drained to bottom glass to get rid of turbidity. Then I laid down fresh damp mulm on both ends of tank 1/4-3/8” thick, took a ketchup squeeze bottle and lightly washed mulm down into top 1” of substrate. Then I let it set 36hr in that high aerobic environment, all the bacteria and fungi in that mulm were supercharged and ready to go by time I slowly filled tank without disturbing mulm layer, planted and turned pump and light on.
The little aquaclear I’m using was specifically placed so output sweeps over top of those tallest rocks/cave, hits bow front glass, creates a down draft and pushes micro currents into subtrate under this 16x8 gravel bed at that low spot at front.
Within 2 days of planting this the 3 single little sprigs of Rotundifolia on each end had rooted in, grown 1.5” and where starting to send out multiple massive sized side runners across gravel.
Within 10 days those runners had completely encircled the whole tank, front, back and sides, sending up new stalks every 3/4-1” or so. Red root floaters had tripled mass and Rotala original stalks had reached surface and where now creeping 3-4” along water surface and branching out. Entire surface of tank was covered with plants.
To say mulm is bad for your aquarium is akin to saying compost and mulch is bad for your garden. Nutrients are recycled and made available for your plants again. Only difference is we have to deal with it more diligently in a closed aquarium environment. Why remove nitrogen and other compounds then have to replace them with a man made supplement. Let nature do what it’s been doing for millions of years and just lightly supplement it as needed.
Only thing I plan on doing to this little tank is add surface extractor, slowly remove about 80% of that mulm and start doing 1gal (16%) water changes. Surface scum compounds are one of main ones that can’t be dealt with in oxic layer I’ve set up in substrate and will be dealt with by physical removal and also periodically running carbon and/or purigen in filter box for couple days at a time.