I learned from last years severe draught and I've increased my rainwater storage, now up to 325 gallons. The two open tubs grow algae and other muck, no issue at all. The only thing I care about is the mosquitos, so my son goes to the pond and catches a few small Blue Gill sunfish and they work mosquito patrol all season till the cold hits. Probably eating some of the algae too but this year we discovered some snails hitched a ride and are now keeping the tubs clean.
I've been using a method of fermenting plant waste in a bucket to create natural fertilizers and improve the soil micro-biome at the same time. It's been effective. I can't imagine algae, aquatic plant life, is going to be harmful.