Those of us who have been in the hobby long enough remember the high esteem in which "mulm" was once held. Sensitive fish could only be maintained in tanks with a thick smear of mulm on the bottom. The smallest of egglayer fry needed mulm to provide them with microscopic food in their first days of free-swimming life. Mulm concealed and coddled eggs; it added magical elements to raw water that transformed it from a simple sterile liquid into a mystical almost-living elixir that kept fish healthy.
No, it doesn't smell bad...and it is loaded with countless microbes that might feed fish or might infect them. It doesn't terrify me, but I sure as hell don't encourage it (as we once did) and in fact I try to minimize it to the best of my abilities. In polite conversation, or in writing, I would likely call it "sludge"...but in the secret confines of my mind, or when in discourse with other like-minded heathens...it's s**t. Not "poop", which imparts a cutesy-poo politeness to it...not "mulm", which gives it an old-timey nostalgic appeal...it's s**t, plain and simple.
Sure, it's got other goodies in it; plenty of degraded plant matter, probably shed scales and atmospheric dust and dead bacteria and stuff...but let's face it, its single largest constituent is piscine fecal matter, i.e. fish s**t. Once it sits long enough, much of it has dissolved into the water, leaving the remainder as the more-or-less inert harmless brown goo we all know and...tolerate...but, hey, let's be honest: it's still s**t. Not fresh s**t, to be sure...but still s**t.
If my dog leaves a turd in some far corner of my yard, one that escapes my immediate notice and is baked by the sun and mummified by the wind...it may be more or less odourless, and it may be the easiest type of turd to flick cleanly into the collection bucket...but it's still s**t.
The rich black or brown soil that gardeners worship, and which covers the land areas of the planet and allows for the growth of plants, consists largely of worm "castings"; that's another example of people being afraid to call something what it truly is: s**t, in this case, worm s**t. People buy s**t by the bag or by the truckload, calling it castings or manure or whatever; the Toronto Zoo does a thriving business in ZooPoo, which is sold in small bags to be used for houseplants...but it's all a semantic game. It's all s**t.
S**t...or whatever you want to call it...is stuff that is cast out from the body of a living critter. It's stuff that the body needs to get rid of. It's neither necessary nor even desirable. It's gotta go.
So, cleaning it out of a fish tank before it has a chance to leach out and dissolve many of the things that make it undesirable seems like the logical course of action to me. I make sure I have an army of beneficial bacteria who actually find the stuff useful, and I rely upon them to do their jobs, but I want a filter or pre-filter that I can get at easily and clean/change quickly enough that I can do it every day. The bacteria can fight over the rest.
