This is somewhat different from the original issue of taking on contaminated water, but one way for "biothings" to get into the water tank is through the vent. Essentially, every time you draw out water (i.e. at the sink), you are sucking in air (plus what comes with it) through the vent. Some people recommend a filter on the vent (even something as simple as gauze), and/or moving the vent to a better location (recognizing that some boats have it in the anchor locker (eww) or outside, or in other locations).
There may not be a pressing need for everyone to change the vent, but if you are commonly finding your tank water to "grow things" over time, it might be worth considering (I'm sure this is location dependent, etc.).
I just looked over a friend's RV water tank setup the other day and although most of it looked pretty good, the vent hose (which leads outside) was absolutely BLACK with mold. Clear hose that you could no longer even have a hope of seeing through.
Another thing is that clear hoses that are exposed to light can accelerate the growth of "stuff" as I understand it.
Maybe things to consider if a particular tank or hoses seem to foul on a regular basis.