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surface scum and substrate bacteria
Surface scum thoughts:
I've had surface scum come and go in my tanks, with or without surface
agitation. I sometimes see it appear after a change in the system and
then disappears again after awhile.
I suspect that the scum develops when conditions change to produce
proteins and lipids in excess of what the tank's bacteria population can
consume. It disappears when the population grows or adapts to meet the
supply.
Bacterial populations in aquariums can develop very slowly. I've seen and
heard plenty of observations indicating that several months can be
required to establish many populations and I've seen changes in my old
aquariums that suggest that some specialized populations of bacteria
and/or fungi could require years to get established.
On that basis, I'd expect surface scums to be fairly common in tanks that
are less than a year or so old. I also see scum appear in established
tanks after something disturbs the microbiological conditions in the tank
- for instance, when the filter is changed, the substrate is disturbed or
the fertilizer regimen is altered, and I expect that is normal.
I don't think scum should be a persistent problem in an old tank under
stable conditions - certainly it isn't a persistent problem in mine.
Substrate microbes:
Natural soils are very dependent on their populations of bacteria, fungi
and other microbes for their activity and fertility. Terrestrial plants
are very much dependant on those populations and I'd be surprised if our
aquarium plants weren't sensitive to the presence or health of bacteria
(and other microbes) in the substrate.
One of the real advantages to using natural, unsterilized materials in
the substrate - like Paul K's soil soup - is that it also introduces
diverse populations of soil microbes to your tank. It might take years to
build a population comparable to what you can get by using soil in your
substrate.
Roger Miller