I do indeed have a crawlspace and I've just come from there. Trying to figure out if I need to reinforce the floor for a 180g or not. Looks like it's in a great spot but that's a discussion for another thread.

I was under the impression it's bad to store water in the crawlspace however there are companies who make water storage tanks specifically for crawlspaces. The crawlspace has several heaters down there set to 10-12c so that's certainly warm enough for the water if I have to use a pond heater to heat the water to tank temp that would be fine.

I thing the main issue would be ventilation. If its vented in a way that moisture does not build up or your containers are sealed so that they cant evaporate into the crawl space which might not have much air flow you would be fine. If you own the place I think I would add an out side vent with a fan that pushes air out and an in side vent somewhere so that heated house air can be pulled down and in to keep any moisture moving out. What size of vent and fan you will need Im not sure. Just like our tanks. Standing water isn't good and neither is standing air. Other wise I think you will be fine.

Don't make the water extra salty. You will get a ton of precipitation and many elements will be all out of whack.

I don't see why you can't store inside...don't you have a spare room? I have my tubs in our spare room which also functions as an office.

this shouldn't be a problem if the water is heated and mixed with a power head.