I have a couple big plastic tanks that I collect rain water with for watering my garden and fruit trees.
Reddish and greenish algae both keep growing on the inside. What should I use to get rid of it? Bleach or something else?
Bleach should work. You also can use “green” siding cleaning solution, and if possible use power washer, that will minimize amount of chemical used. Rinse well after.
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If you’re just watering plants with it, I see no need to add any algaecide…the plants don’t care!
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I was afraid it would affect my plants
Unfortunately i can’t reach all surfaces so the algae keeps spreading from the areas i can’t reach
Then it is not that easy… You basically need 10% solution of the bleach to kill it. For a large tank it is huge amount of bleach, if you need to fill it full. But only reason to be afraid of algae is if it grows in your hydroponic tank. Otherwise it is fertilizer… The Benefits of Algae as a Fertilizer – AlgenAir
Another suggestion, as algae needs light to grow, cover your tank and if it is white paint it or even better wrap to cover from sun.
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I called it algae because I assumed it was. If there any way it’s something else? There’s a thin layer of a red film on almost the entire inner surface of the tank
It’s probably an algae - some are red-brown in color. But… there are invariably environmental bacteria growing in there as well. And… eventually, invertebrates like water fleas, water boatmen, mosquito larvae, etc. will find their way there.
Again, the plants don’t care if their water is coming from a environment with a diverse population of organisms growing in it.
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My rain barrels are green, so I do not really see the film, but I am sure it is there… It is slim 
. Never had problem with watering.
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As Galina’s says, cover it and algae will die. An ounce of prevention and all that.
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Algae acidifies the water. I guess, depending on your soil, and what you’re growing, could be good or bad. Great if you’re growing blueberries.
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I have heavy clay alkaline soil. In fact my blueberries are in pots because they do horribly in this ground
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1 vote for something else.
Freshwater Snails. (thats what they do for a living).
Shotgun style answer here.
I do like the snails idea, and concur that the algae are not hurting anything. Most do prefer sunlight.
I have kept fishtanks for a long time. New tanks always go through a balancing period. There is no reason to expect different from a rain barrel. You’ll end up with a slick on the surface of the good bacteria. More surface area means more bacteria. More bacteria often means less algae. So, add a few rocks to the tank to increase the surface area it contains, or a layer of coarde gravel.
Red is generally cyano-bacteria. This one is less fun to keep around, but it tends to go away in about of month from set up. They seem to really thrive in new aquariums, especially those with a lot of silicone or playground sand as substrate.
In the old days, when silver dollars were really silver, we would put one or two in the drinking water barrels to take advantage of silvers anti-bacterial properties.
My rain barrels have a layer of the rocks I dug out of the yard in the bottom to comat the winds. One of them has a pile high enough that the birds who drink from it can stand in one spot. In the summer, they also usually have guppies and duckweed from my tank, as both reproduce like the proverbial rabbit. The guppies eat the mosquito larvae. Another sits next to comfrey, which contributes random leaves that renders it always a weak compost tea.
Long story short: unless you’re drinking from it or it is clogging equipment, don’t worry much about it, and nature will do its thing, probably to your plants’ benefit.
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