Don't overthink it unless you know you have a suppressed immune system. We have motorhomed in 30 countries always filling from local sources including roadside springs and rarely treated the water (with standard unscented bleach) and have never in 10 years suffered the slightest health problem due to water safety. Countries include Mexico, Turkey, Morocco and most of the Americas.
About the only time we have bought water to drink was in Baja Mexico and a couple of places in thge USA where the water was perfectly safe, but tasted terrible because of mineral content.

The biofilm you saw could have been very fine clay deposited out on the walls of tanks and pipes and even if it was organic, was likely algae or other benign growths.

If you are concerned, it is quite easy to sanitise the complete water system using normal chlorine bleach at a certain concentration - fill the tank and run each hot and cold tap until you smell chlorine and then leave the whole thing for a few hours and then drain and refill. Water itself can be treated if suspect using a much lower concentration of bleach. There are also colloidal silver products you can add to the tank as you fill it that are claimed to sanitise drinking water.

If you insist on no chemicals then never ever fill from any municipal water supply and install a reverse osmosis system and just use that water. Or find a nice little mountain stream high up in the mountains and hope that there isn't a dead deer in the water upstream and that the water does not flow out of a spring with high levels of argon or dissolved heavy metals or ??????.

Then of course you will not want to breath the air anywhere in most of industrialised Germany or anywhere within 100km of most major cities or towns.

--
Tony Lee
Photos at https://picasaweb.google.com/114611728110254134379
Travels map at https://spotwalla.com/tripViewer.php?id=5cfc50ef7ac22ca2d&hoursPast=2400&...

Um diese sich leider etwas einseitig entwickelnde Diskussion zur Trinkwasserhygiene zu stoppen: Wir haben uns entschieden!
Nach Rücksprache mit dem Wasserwerk wurde uns ein 2-Komponenten-Chlordioxid empfohlen, welches in einer für unseren Tank errechneten Minimaldosis dem Wasser (ggf.) zugefügt wird. Über einen tatsächlich notwendigen Einsatz werden wir von Fall zu Fall vor Ort entscheiden.

Wir waren zwei Jahre unterwegs und ich glaube Javelwasser ist das billigste und nützlichsten was du einsetzen kannst$

1.5 mL Javelwasser (2.5 %) auf 100 Liter Wasser

und alles ist gut. Wir haben das Wasser dann nur im Notfall getrunken sondern immer grosse Wasserflaschen vor Ort gekauft. Aber das Wasser war absolut trinkbar. Einfach im Tee war es nicht so fein.
Gruss
Weggu