We have an above ground pool with about 6 inches of water that does not drain. The drain is located above the bottom, to keep water in it and weigh down the pool during the off season.

However, the still warm water is the perfect breeding ground for mosquitos. This is a problem during the advent of warmer weather, before the pool is used.

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It is a salt water system, and the pumps & chlorinator obviously cannot be operated with the water drained to this level. Once the pool is operational in the hot season, the conditioning and filtering prevent this mosquito problem. So this is really only a problem when the Spring weather is warm enough for breeding, but prior to starting the pool for the Summer.

Before I start-up the pool to fill it, I drain the water with a sump or water feature pump and then wet-vac it dry. This is something I do once at the beginning of the season, and takes a few hours of work. As measure to prevent mosquitos it would not be so good, since the bottom will fill with every rainfall and I'd have to start over.

What can I do to prevent mosquitos breeding until I start-up the pool for the season?

  • 3
    Fit a domed cover and then keep it dry.
    – Solar Mike
    May 16, 2021 at 18:34
  • 1
    Add a little soap to the water, and the larvae cannot stay at the top to breathe or is it oil? vegetable or other??
    – Jack
    May 16, 2021 at 20:57
  • 1
    @Jack it is oil
    – jsotola
    May 17, 2021 at 4:37
  • @Jack I second this. We throw a few drops of oil into a stash of old tires to keep mosquitoes down. I was told the larvae cannot emerge/breathe with the skim of oil on top.
    – Aww_Geez
    May 17, 2021 at 14:51
  • 1
    Don't put soap or oil in your pool - that will just make a colossal mess that will require a ton of water to wash out and clean before you can fill it and start filtering.
    – J...
    May 17, 2021 at 19:07

5 Answers 5


First point : Chemical warfare is the tool of last resort for pool maintenance.

Sometimes you end up in a crisis and you need to deal with it, but if you look after the pool correctly it should be absolutely unnecessary. Chemicals are a double-edged sword. They can get you out of a mess, but they come with headaches of their own. If you can avoid letting the pool get this bad in the first place it's altogether the better option.

This is a problem during the advent of warmer weather, before the pool is used.

The error here is to think of filling the pool as something you do when you want to swim. If a pool is of such a size that it cannot be drained of water then it is something you need to care for year-round, and treating the water is as much for the pool's sake as it is for yours. If the pool requires some water to remain in it all year around then it requires maintenance all year around.

Once the water temperature rises above 10C(50F), the water becomes a breeding ground for insects, algae, etc. You need to fill the pool and start treating the water when this happens - not simply when you want to swim. The pool needs to be opened sooner - once the water is chlorinated and is being filtered with the pump it's fine. So you need to start doing that before biology has a chance to take over.

If you really don't want to open the pool when the temperature starts rising, you need to otherwise keep it covered and chlorinated. The cover will prevent the sun from burning off the chlorine, but you will need to monitor the chlorine levels or algae begins to grow, and since this is a lot easier to do and manage with the pool filled and circulating daily it just makes more sense to open the pool and start treating it once the weather becomes favourable for algal growth and insect infestation.

In the long run, this saves you on chemicals, cost, and nasty cleanup time (scrubbing out algae and slime) and it increases the usable life of your pool.

Before I start-up the pool to fill it, I drain the water with a sump or water feature pump and then wet-vac it dry, but obviously this would only be good until the next rainfall.

If you open the pool earlier, before it grows into pond scum, you can keep that water from last year, for example, and save yourself the trouble of scrubbing all of that filth out. Same thing in the fall - keep the pool open a few weeks later, until the water temperature drops below (10C/50F) and give the leftovers a good shock before you close up. You can have clear, clean water sitting in the pool over winter and ready to go come spring.

In colder temperatures when the pool is not getting much use you don't need much circulation to keep the water clear - a minimum of chlorine and a few hours of circulation is all you need. It will depend on the size of pool and pump, but whatever your pump's throughput is you can calculate how long it takes to circulate the full pool volume. As long as you run about a pool's volume through the filter per day that's plenty. A few hours is usually enough. Use a timer.

The energy and chemicals costs of keeping the pool open longer are small compared to the work they save you in having to un-swamp and hard shock the thing every season. My pool has been open for the past seven weeks and I've spent perhaps $20 of electricity and used 2kg of chlorine (in-ground, 80,000L (20,000 gallon)) - for a smaller above-ground pool your costs will be even less.

You'll want to be very careful (ie: don't do it!) about adding additional chemicals to your pool. Soaps, oils, insecticides, metallic algecides, and other additives are not good for the pool, the pump, filter, or your chlorinator. If you add these things to the still water, you again have a big cleanup job to get all of that stuff out of there before you start up the pump and filter. If you don't clean it, it will end up in your filter and can cause damage to the pump or the other equipment.

  • 1
    Also once temperature is low, a few hours of filtration a day is more than enough (use a timer). The amount of chemicals needed is also much less. It really is easier to run the system as long as the water is not frozen.
    – Olivier
    May 17, 2021 at 18:52
  • @Olivier Yes, absolutely. I've added that to the answer.
    – J...
    May 17, 2021 at 18:57
  • 1
    @P2000 Indeed, and not just mosquitoes - I see in your photo that there's a lot of algae growth in there also. Opening earlier solves both problems.
    – J...
    May 17, 2021 at 21:06
  • 1
    I went with this solution: start the pool early (although we had warm days, it's still early here). Added benefit: during cleaning the sun doesn't cake on the algae and leaves. Thanks for the nudge.
    – P2000
    Jun 8, 2021 at 17:50
  • 1
    @P2000 Glad it worked out. My pool has been open since Apr. 10th and the water has been clean and clear the whole time - it really makes life easier! I think this week is probably the first time that the pool is now warm enough to properly swim in. Secondary benefit is that you can look out your window and see a nice blue pool instead of a festering pit or even just a cover.
    – J...
    Jun 8, 2021 at 18:16

Mosquito Dunks are a doughnut shaped product for exactly your problem. They contain a bacterium toxic to mosquito larva but not toxic to humans, pets or birds. They will only kill larva.

Any adult mosquito travelling by will still be available to bite you. You might also check out designs for bat or bird houses to encourage some natural predation.

  • 1
    AKA ; BT............ May 16, 2021 at 18:53
  • Natural predation would be great during the season. How many bats or birds would you need? Of course "it depends", but for around the pool, are we talking 1, 10, 100? Nice tip. I'll first try the BT/Dunk for the standing water.
    – P2000
    May 16, 2021 at 20:00
  • 1
    We use BT, it works well. I buy it in powder form, mix in with water, and spray the pool. 5 minutes every week, no mosquitoes.
    – bobflux
    May 17, 2021 at 12:21
  • @bobflux do you do that for a full pool or do you have a similar "bottom 6in" problem? How much do you use?
    – P2000
    May 17, 2021 at 21:03
  • Half-full pool, depends on how much it rained. I use the amount it says on the can, about one tablespoon powder in the sprayer. BT is nice because it's not toxic to humans, fish, etc, only kills larva. But you got to do it every week. I bought a 500 grams can years ago, it lasts forever.
    – bobflux
    May 17, 2021 at 21:32

Copper metal.

Copper discourages mosquito larvae. I used to think that the copper was directly toxic to the larvae but experiments showed this was not so - adding copper (as metal; a piece of pipe) to a fish tank where larvae were already living did not kill them.

Copper in standing water however reduces algae growth and algae is what the mosquito larvae eat. An outdoor fishtank with a piece of copper pipe in it from the start harbors few larvae or algae.

I keep a coil of thin copper pipe and put it in the aquarium if the water gets green. It works fast. If I leave it more than a couple of days the fish stop eating but they perk up a day or 2 after it is out.


The other option is goldfish. I tried this at a different year from the copper experiments. I had a fish aquarium outside which had filled with rain and then with mosquito larvae and it seemed a shame to waste them. Two 8 cent goldfish made short work of those larvae and then sustained themselves on whatever else fell in the water over the rest of the summer. Two goldfish would be cheaper than a piece of copper if you don't have any pipe scraps. I used 2 because just 1 seemed lonely.

For efficiency the copper works fine and you can take it out and use it year after year. The goldfish were more fun.

  • 1
    That's a lovely idea: Turn the pool into a goldfish pond in the off-season! Add some algae eaters and you are all good. May 17, 2021 at 11:52
  • 8
    Just a minor nitpick, copper is toxic to pretty much anything from algae to humans. But a piece of copper metal will only very slowly release toxic copper ions, so there's fair chance that the resulting concentration simply isn't high enough to kill the larvae. If you added some copper sulphate to the water, the larvae would promptly go belly up. (The water wouldn't likely be safe for humans afterwards, so don't.)
    – TooTea
    May 17, 2021 at 12:11
  • 6
    +1 for "I used 2 because just 1 seemed lonely.". Priceless.
    – WernerCD
    May 18, 2021 at 1:03
  • 5
    Make sure your goldfish are compatible with your saltwater pool. It'd be a shame to waste those 8¢ goldfish...
    – FreeMan
    May 18, 2021 at 13:02
  • 2
    My beautiful shubunkin and my cute goldfish were eaten by the neighbour's new cat. OK, I can live with brown Bitterling. I will get new pretty fish when either that cat dies or I find a way to keep cats away that still allows little birds to drink and bathe. To ward off herons we have two bronze sculptures, because herons do not like company. (We got two because one would be lonely.)
    – RedSonja
    May 19, 2021 at 13:39

It is a salt water system, and the pumps & chlorinator obviously cannot be operated with the water drained to this level.

Just in case you're not aware, the salt water isn't what keeps your pool clean. The chlorinator converts the salt to chlorine. You've got two problems there, in that the green stuff is algae (which is is a pain to rid yourself of once it gets going strong).

Go buy some liquid chlorine (or basic un-enhanced bleach) and start shocking the tar out of that standing water until it is no longer green. At that point, the mosquito problem will be gone as well. Liquid bleach plays nice with salt systems, just be aware that after this is done, you might need to check the pH levels before refilling the pool (as this will probably make the remaining water acidic).

Something to consider for the off-season would be a pool tablet float. It will add some chlorine slowly to the standing water when your pump isn't running.

  • Yes, chemically treating and winterizing the bottom 6in is a good idea. However, the winter rain will flush it almost monthly, so I'd have to see what would be a good release rate, or perhaps add the chlorine early spring when it's warmer but does not rain as much anymore.
    – P2000
    May 17, 2021 at 21:02
  • @P2000 The key is to take note when the daylight gets long enough to warm the water and start the serious stuff then (I'd start the tablet float as soon as coat weather is done). Every time I've let my pool get that green, it takes a ton of effort to get it back to normal. Mosquitos are seriously the least of your problems here.
    – Machavity
    May 17, 2021 at 22:10
  • Depends on your climate, but mosquitos are only a problem in summer here.
    – RedSonja
    May 19, 2021 at 13:41

Any standing water attracts mosquitos so the first step is to drain all water. If draining is difficult, add BT ( bacillus thuringensis ) , a common insecticide that only affects insects. Other insecticide could be used but may require reapplication, and may require removal before swimming.

  • BT might be the only solution. Draining is not possible because of height of drains and I believe intentionally sp for the reasons mentioned in the question (weighing down). Removal should be no problem since pool is vacuumed and scrubbed before fill.
    – P2000
    May 16, 2021 at 19:54
  • 2
    Draining is possible. I'd use a sump pump or a wet-dry vac. No way I'd want to swim in that, especially after scrubbing.
    – isherwood
    May 17, 2021 at 12:51
  • @isherwood, yes under this proposed treatment I'd still drain, scrub and wet-vac.
    – P2000
    May 17, 2021 at 20:45
  • After some more reading based on this answer, I'd like to add that we need BT (as mentioned here), not BTK. So called "Mosquito Dunks" contain BT. But note: "More than 20 varieties of Bt exist. The "kurstaki" variety (Btk) is used to control caterpillars."
    – P2000
    May 18, 2021 at 22:05
  • BT is active in alkaline stomachs ; that is insects including caterpillars. It is not active in acidic = mammal and other stomachs. My guess is the varieties are more to do with sales - "ours is better than the others". May 18, 2021 at 23:24

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