2 incidents from this year with the mini. One was Santa Maria Spring on the Hermit Trail in the Grand Canyon. The Spring trickles into a tank that gets covered in algae. I poured the water through a coffee filter into the dirty bag before running through the filter. Water was fine. I didn't really need the water, but I wanted to see how this worked.
Did the same thing a few weeks ago from a ridge top pond that is filled with algae, leaves and mosquito larvae. Not an ideal source, but the trail to the ridge is steep, steep, steep, and weight reduction is a good thing. We got water, and it was fine.
I would take the syringe and back flush after every use out of that pond. Boiling would be good, too. I'm only addressing sediment and algae, not pathogens, but having worked on many dairy farms and cattle ranches, the poo factor is off-putting but survivable. Not a scientific response, just one of practicality. Maybe someone has more data oriented experience and knowledge about getting water from stock tanks.