because of the porosity you mentioned, I would treat the water as any other suspect source...you can save a lot of water if you flush by adding non potable water directly to the bowl... try it first and pour slowly...

My first go to choice after the dedicated storage is the hot water heater...if it's elevated it's much easier to access...cut a section off the female end of your garden hose long enough to reach a fill station, clean the critters out the section, and put a female repair end back on the outside section

Just received my Sawyer .1 micron 5 gal bucket kit, you guys really need to look at this thing.. just over $50 at Adventure Safety Products...