Homemade Catch tank

Not sure if this is the right place or right forum to ask, but only one I can find so far. I acquired a 275 gallon white poly tank and decided to use it for a rainwater catch to use around the yard. Problem is after a couple days the algae buildup or something stinks too much to use. Is there an easy fix? I was thinking because it is clear/white the bacteria is building and cause the smell. Can I simply paint it black or dark and stop this? Or is it futile and I should start w a different setup? I’ve tried adding a little chlorine to combat but if I add enough, then I can’t use it on the yard.

First try to limit sunlight with placement, which also helps keep water temp lower in warmer months. We have two at the north side of our garage so they only receive a bit of sunlight in the morning and evening. You can paint or cover the tank with dark tarp and it will stop growth. One of our tanks is painted a matte brown. The other is still white and I haven't had an issue but just really started using it last fall and we are probably just warming enough to allow growth. Until a few days ago the inside was slowly melting  mass of ice so it may be too cold for much.  I plan to shock it with bleach and paint similiar to the other. This is the third season I'm using that one with gravity feed to the garden and it's saved us numerous times. Consistently using the water helps prevent things from stagnating as well, if you know a good rain is coming cycle the water out.

Do you know what was in the tank before you got it? Is it possible that there could still be some organic residue left over which is causing a Bloom of biology?
A rewashing might help.
If this is anaerobic stink, finding a way to get airflow over the water inside might help.

Is there any extra organic material getting into the tank with your rainwater?
(IE, chickens on the roof pooping, tree fruits washing down?) Any extra 'fuel' could be causing bacteria growth.

Mosquitoes? Add a couple mosquito fish or guppies.

I recently joined an aquarium page. Modern aquarium designs work a lot more organically. Getting biology to work with you in the tank. They frequently mention 'cycling' a new tank. It's getting the bacteria and biology in the aquarium into equilibrium before adding fish. I have seen people mention fuzz growing on tank decorations until this all settles out. I wonder if your rainwater tank might be doing the same thing?

I would paint black, then cover it with something reflective, to kero the water cool.
You might also try adding charcoal to the water.
I float pieces of char in my chickens water and it seems to keep alge growth down.

A solar powered fountain could aerate the water and thereby fight the stink.

Thanks for the replies. To answer a few of the questions, the tank gets rainwater off an asphalt carport roof. It’s not in direct sunlight , it’s kinda blocked most of the time by tree shade. We are warm most of the time in Virginia. I can’t really access the tank to put anything in it except through the 3” pipe that the water can access. I doubt mosquitoes can get to it and no way I can get movement other than dumping. I’ll have to try the suggested painting dark and see if I can get a way to add charcoal here and there and see what happens. Thanks for the replies, I’ll keep checking back.

Hi Thomas,
Do you have what is known as a "first flush system"? This is just a way to get bits of shingle, leaves, bird poop, etc off to one side before starting to fill the tank. If you don't already have one, it might help keep organic materials out of the water. The more organic matter there is for bacteria to eat, the greater the growth.

On the other hand, since you are planning to use it "around the yard" I'm going to guess you are planning to water trees and gardens with it? If that is the case, I don't think it would be that big of a deal, other than the smell. If you were going to drink it then you would need to be more stringent.

Matt McSpadden wrote:Hi Thomas,
Do you have what is known as a "first flush system"? This is just a way to get bits of shingle, leaves, bird poop, etc off to one side before starting to fill the tank. If you don't already have one, it might help keep organic materials out of the water. The more organic matter there is for bacteria to eat, the greater the growth.

On the other hand, since you are planning to use it "around the yard" I'm going to guess you are planning to water trees and gardens with it? If that is the case, I don't think it would be that big of a deal, other than the smell. If you were going to drink it then you would need to be more stringent.

I have something similar to this… https://www.amazon.com/Atlantic-Water-Gardens-Downspout-Harvesting/dp/B004SI1DHE?asc_source=01HFY6QAA19TPJKTPW1MKMF1KH&tag=snx87-20


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Hi Thomas,
I'm not familiar with that particular one, but it says it does the first flush, so that is good.

William Bronson wrote: I would paint black, then cover it with something reflective, to kero the water cool.
You might also try adding charcoal to the water.
I float pieces of char in my chickens water and it seems to keep alge growth down.

A solar powered fountain could aerate the water and thereby fight the stink.

Great idea with the charcoal, I'm going to try that out this season. Maybe I'll leave my tank white for observations purposes.

I'd mentioned above that the unpainted tank had no growth yet and in just a couple nice days that has changed. Obvious pink algae forming. It's only got around a hundred gallons left now, I'll be shocking it today.

I really like the idea of mosquito fish but we cannot get them anywhere locally. Also curious that they wouldn't find their way out with the flow?

I wonder if your roof material is not satisfactory in the first place?
Alos algea is usually green and does not always small, something may be feeding it.
I have 200,000 L stored at my farm.
I actually have an IBC I use to top up a bird bath, it is in full Australian sun, with a first flush unit and I have never had algea in it.

I painted the tank a dark green. It’s not in direct sunlight but after a couple days it had some smell to it again. I added an ounce or two of bleach. It’s normally between 100-200 gallons unless we get a heavy rain. Going to check if the bleach helped today. Wondering if a vent at the top would help at all. The water goes into it a foot and a half from the top and that’s the only air vent. Wonder if I can ad a small vent at the top of it to keep some of the heat out and help. 🤔

I collect roof water in the tropics in stock blue and gray plastic tanks. They are not painted and some light comes through - though they are located in the shade of vegetation. Some light algae film grows on the tank walls but it does not cause any odor. We have a first flush tank so roof drainage goes through that tank first and only reaches the big storage tanks via overflow of the flush tank with a screen on the overflow pipe. If a tank of rainwater has odor, it is surely debris in the bottom of the tank rather than algae, imho.