I don't know what A. Smith intends to use the rainwater for, but I installed a big tank (about 6 ft high) from the local farm co-op more than 15 years ago that I intended for watering a large garden. It's translucent white poly, which I expected would result in much algae if I didn't darken it. But I was too busy with other projects at the time to do anything with it and left it uncoated and uncovered. However, with only trees to shade it about half the day, I was pleasantly surprised to see that it never did develop significant algae -- only enough to give the water a green tinge.
In the intervening years, I've used this tank to hold pond water pumped out of my fish pond for spring cleaning, so it's well-inoculated with microorganisms and has a thin layer of sludge on the bottom below the drain outlet. (I've tried cleaning thoroughly a couple of times, but that's an awful lot of work.) If I intended this water for household use, I'd certainly screen out debris near the inlet (and not use it in pond cleaning
) and would filter and probably disinfect the outlet water, but I haven't needed anything beyond "roof washing" before connecting the main fill downspout to capture a rainfall.
Here are a couple of pictures taken this morning (with the main fill and hatch cover removed):