I've got several hundred gallons of rain water that has got algae and mosquito larvae growing in it and I'd hate to waste it if it could be used. A search of this forum has led me on a goose chase. Can you water anything with it? Fruit trees, vegetables, compost, anything useful?

Thanks

I would have no problem watering trees and such with it, I do here often. You could also drop a few feeder goldfish in to clean it up a bit.
I have heard but do not know that putting some cooking oil in will float and smother...if you try that let me know how well it works. I use fish in my big tanks.

I'd either add some BT for mosquitoes or some gambusia (mosquito fish) and use it as you will. No harm watering plants with the stuff in my opinion, maybe lay off the seedlings and anything rare in case you get anthracnose or anything weird like that.