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using mulm versus starter cultures

Warning: rather long story.

A year ago I tested a few of the so-called starter cultures that are being
sold to accelerate the start-up phase of biofilters in aquaria. This
included three liquid, one freeze-dried and one powder (company did not
state that it was freeze-dried) product. All of these products claim that
they are able to remove nitrite. What is rather strange is that some only
mention nitrite and not ammonia.
None of the liquid products showed a significant decrease in start-up time
of the biofilter compared to a system without inoculation (blank). Start-up
times of the biofilters took between 4 and 6 weeks.
Same story for the powder product.
The freeze-dried product on the other hand did show some clear activity:
after 5 days, all ammonia I added to the system (10 ppm N) was removed, BUT
no nitrite oxidation occurred until day 28... So also this product did no
pass the test.
To check whether there were any viable nitrifying bacteria present at all in
the liquid products, I inoculated a specific medium for nitrifiers with the
products and checked for activity. In that case there was some activity, but
it seems that there is only a very low concentration of nitrifiers present
in these products (less then 10/mL, which is very low). If you keep the
very slow growth rates of these creatures in mind, it is NOT worthwile to
buy these liquid products I tested.
Therefore it seems to me that using mulm from an exisiting biofilter is more
interesting, allthough you will have to take care that you add enough mulm,
to guarantee a good supply of nitrifiers. Together with the nitrifiers you
will also add a whole bunch of other bacteria, mainly heterotrophs that will
also help you to keep you water clear.

A liquid product should contain quite high concentrations of nitrifiers to
be effective as an inoculum. Storage is probably one of the main problems.
Therefore a good supply of fresh mulm from a healty aquarium is probably the
best thing you can do.