Hey,
I think everyone pointed interesting aspects.
But what do we want? – We want acid water, no actually sterile water. The content of ions are important, but the density of bacteria also matter. Peter and others tested that before. So we use acid and tannins to reduce bacteria. Usually we don´t want black water because of its color.
So yes, bacteria are everywhere and able to live in way more habitats than fishes, but the peatbogs exist for a reason: the bacterial and fungal activity and degradation is reduced.
Back to the topic: I think there are a lot of parameter that influence the efficiency of your filter.
As long as the current is not too strong your filter wont harm your fishes. You will make your experiences and decide on your own whether a filter is good for your fishes.
@beutho: Have you any evidence that Archea have a significant impact to fish tanks? (I´d just like to know)
Best,
Bennie