I don't consider mulm the least bit controversial. It's always been standard procedure for me and many LFS's. Adding it works. I lost count as to how many times I've set tanks up with it (just about every tank I've ever set up all my life has had it added). I told folks to do that when I was a kid working at a LFS years ago. I was told to do it by an old salt.
My substrate can be up to 12-16 inches deep in large tanks and 7-8 inches in rear spots in regular smaller tanks and slope down to 1 inch in the front.
I've never had a single substrate issue except once when a cable heater cooked an Aponogeton bulb and it died. I vacuumed the gravel around the area and that was that.
Folks that move slow bulb (Crinums, some Aponogetons, Lilies/Lotus etc) plants around too much can suffer this fate.
But these plants do quite well in anaerobic substrates, that where they live in nature, most aquatic rooted plants for that matter.
Adding mulm does one primary function, adds the good bacteria and instantly cycles your tank of which the filter and the substrate possess the largest colonies of bacteria. Lots and lots of surface area. Roots invade these regions and the bacteria have associations with the plant roots. This speeds up this bacteria/plant root colonization.
Aquatic plants release _large amounts_ of O2 to the areas around their root zones(this is how they live in anaerobic waterlogged soils).
They also relsease H+ to add reducing power to absorb nutrients.
Peat helps this process(H+), taking some load off the plants till the tank is established and going well.
Both of these additions simply give the tank a jump start.
Adding peat adds some reducing power to the substrate that will develop as the tank/substrate ages anyway. This allows the nutrients to be more available for the plants.
A 20 gallon tank will get about 1 liter 2 liter of decanted mulm.
One good sized handful of _ground_ peat.
Top with flourite or onyx sand, 2-4 inches. You can slope the gravel front to back or side to side etc.
That's it.
You can dry the mulm out and add that also. Fresh is _much_ better. I've dried it in the past and added it. Too much work
Go down to a friends, LFS and and bring a bucket and vac. I'm sure they will not mind a nice gravel washing for free.
Let the bucket sit for 5 minutes of so, pour off the clear liquid.
Save the goop. Add it it right away to the substrate and then get the tank set up and filled. If you need to store it, keep it in a dark/warm place.
Save a little to feed into the filter, that will instantly cycle the filter(or you can loan the filter out to place where you plan on getting the mulm for a couple of weeks if you have the time etc).
Rinse flourite /onyx sand only 3 times in the bucket(1/2 a bag at a time). Add that to the tank, it'll be dusty still but it will be dusty after 10 washes, so don't worry about that, the tank will clear fast. Just do the three washes.
Now you are done.
For non CO2 tanks I add 4x the mulm, 4x the peat. The flourite is at least 4 inches deep.
Regards,
Tom Barr