As far as I know heterotrophs do not live for a long time in big quantities in a tank as they have not much optimal places to live in (mostly thing layer of mulm only). This is why it is recommended to put in an overstocked tank heterotrophic bacteria cultures regularly.

HMF is an excellent base for heterotrophs, so tank will have constant supply of them to mineralize silt in a tank, not only in a sump preventing build up of non mineralized organics ("bad mulm" as i say). This is very good.

Tom, what is the size of a bacteria?
Why they use mostly 200-300nm EstroSieves, not 50's ?

naman