With "checking algae ...

With "checking algae formation" do you mean checking if it is producing toxins? If you are aerosoling this water or using it for agricultural or  drinking water purposes, it is essential to insure you are not bioaccumulating cyanotoxins or BMAA in your media, drinking water system or in your food supply. Elisa testing is a viable technique to provide you with a rough answer on the cyanotoxins. You will need to find a research laboratory to find out if you have BMAA in your algae blooms.

If you wish to collect rainwater for drinking water purposes it is essential that you do not permit any algae growth in your collectors. As others have said, a dark container with no light penetration is required. If toxic algae is suspected you will need to ensure water is ultrafiltrated to remove as much of the organic matter as possible, before the water is put into storage (this is a sound practice even if you don't have existing algae problems). You may also need to chlorinate that water since this is one of the very few ways to neutralize algae toxins.