We installed two 2500 gallon tanks last fall. One tank fills from the back rain gutter, the other from the front. 55 gallon first flush diverters are used, one for each tank. The roof is asphalt shingle. The tanks are opaque (dark green). We are at the 5500 foot level in Sequoia National Forest in California. It’s been hitting mid to upper 80s for 3-4 weeks and both tanks are in the hot afternoon sun.

We began using our stored water this weekend vs our community well system. The water had an odor to it and appeared somewhat green after running a tub full. After pulling the lids and running my hand along the inside of the tanks under water I noted thin green slime.

Is covering these tanks with a roof the only alternative to preventing this build up in the future? I am assuming that painting them black would have little to no effect in that the water in the tanks was as warm as bath water. How much bleach would one use to purge each 2500 gallon tank until he got them covered?

I have both black plastic and grey fiberglass tanks that have been in use several years. Both were sold as “potable water” tanks suitable for rainwater. I’ve seen on evidence of algae though all my tanks are shaded by trees. It is my understanding that algae must have light to develop. Perhaps your tank is not really opaque. White PVC pipe is not opaque and algae will grow in it. Paint may not cure your problem but it would have to help. I would use white or aluminum. Maybe mobile home roof coating.

I’ve put bleach in my tanks just “on general principles”. I find 1 gallon in 6000 gallons is way too much. I now try to put in about 1 pint per tank whenever the tanks go from near empty to near full.

Will look into the mobile home roof coating. Both tanks are in the hot afternoon sun. I may have overdone the bleach, I put a quart in each 2500 gallon tank. Sounds like 1/2 of that would maybe have even been too much. I did read somewhere that if you can smell the bleach you used too much. So far no bleach odor. I sure thought the dark green tanks were supposed to be opaque. Another possibility may be due to the partial filling of the tanks last fall with community well water. However that water never has an odor. I’m sure this odor is from the storing of the water but have yet to determine if the odor is related to the algae or another issue.