The United States EPA has only approved one treatment for mosquitoes in catchment tanks – using an oil that is (apparently) no longer manufactured (Agnique). There are no other approved treatments. (Per EPA response to my query via email, 12/31/2015).
Therefore, this page will review recommendations by the World Health Organization.
The University of Hawaii College of Tropical Agriculture and Human Resources publishes a guide for catchment systems that is a must read: Guidelines on Rainwater Catchment Systems for Hawai`i.
A tight cover
The first line of defense must be a physical barrier – a tightly fitting cover, or a tank with a solid roof (plastic or concrete). Beware that even well made covers often allow channels that mosquitoes can use to get in and out – especially along or inside the overflow pipe.
It takes very strong bleach concentrations (around 200 ppm [Jacups et al: 2013]) to kill larvae so it is not much use as a response to discovering a tank full of wrigglers. You would need to add many tens of gallons of household bleach per thousand gallons of water. Not acceptable. If you find wrigglers, use BT (see below).
However, what a lower concentration of bleach does is kill the bacteria that the larvae feed on. If you do this, the larvae will starve. In Hawaii, we have to worry about leptosporosis, so everyone should be adding bleach or chlorox tablets to their tanks at regular intervals. This will help minimize mosquitoes by starving them.
The CDC published research [Barrera 2004] showing that a concentration of 2 ppm would kill the first stage larvae in 48 hours. This is about 1/2 cup of bleach per 1000 gallons. However, you must add bleach regularly (monthly, or more often in the rainy season) as the chlorine evaporates or is diluted, and these results are for clean water (not full of organic debris like leaves). Follow the recommendations in Guidelines on Rainwater Catchment Systems for Hawai`i.
I have seen catchment tanks with completely failed covers that did not host larvae because the owners religiously added chlorox tablets.
Bacillus Thuringiensis is a bacterium that is used in organic gardening. When larvae eat it, the bacterium produces a toxin that kills the larvae. It is very effective and you will see results in a day or two. It has no effect on mammals. You can get Mosquito Dunks at the hardware store – just follow the directions, and add the correct number every month. (Mosquito Bits, also BT, do not last as long as Dunks).
Chlorine in the water tends to kill bacteria, including BT. If you do not chlorinate your water, BT is a good alternative.
Yes, guppies in your tank. You can also use so-called “Mosquito Fish”, sold at various pet stores and Feed Stores. This is the preferred method for the WHO, at least for 100 – 500 liter tanks (100 gal). If you are worried about guppy shit, it’s a valid concern. However, if mosquitoes can get in and out of your tank, so can geckos, snails, slugs, rats, and who knows what else. And the birds shit on your roof. So guppies don’t sound that bad. And your filters and UV light should take care of any problems. This is the most environmentally friendly and organic method.
Guppies cannot live in chlorine treated water, and therefore in Hawaii where lepto is a concern, should not be used for drinking water (or showers, etc).
Do not release guppies into natural water systems, they are a potential invasive species.
What to do with failed tanks
If you have to deal with a tank full of organic debris that you cannot drain, then use BT and/or methoprene (Mosquito Torpedoes or Altosid). Chlorine tends to bind to the organic material and be unavailable to kill bacteria or larvae. Don’t drink it because of the risk of diseases like lepto.