For my setup, I had rain barrels gathering rain from my greenhouse gutters, then that rain feeds into my fish barrels INSIDE the greenhouse. All barrels are black Rubbermaid garbage cans and the water stays MUCH warmer than the ambient air temps outside:

Nice, good to know. I'm still wondering, though, whether a 300 gallon tank will provide ample water pressure with a bottom connection, or if we'll need a small pump--and if so, whether anyone recommends a particular model. Thanks.

if you plumb a valve into the bottom of the barrel that you can hook up the cold water supply to then while you fill the barrel with cold water from the bottom you will get warm water coming out the top. at least thats how a water heater works....

Yeah, with my 150gallons, I don`t get a ton of water pressure from the rain barrels. Not enough to run a wand or a sprinkler. But the green hose does fill the 3 fish barrels quite quickly. I use the fish barrels for hand-watering.

TheRipeTomato: just to clarify, you *are* getting enough pressure from those Rubbermaid "fish barrels" to hand water with a wand? I'm confused, sorry--it seems like you're saying you have a 150 gallon barrel that's not providing pressure, but that you're hand watering with the Rubbermaid bins (which look smaller than 150 gallons). Do you have a pump on the Rubbermaids? If so, what exactly do you use and how are you hooking it up? Sorry for all the questions and thanks in advance.