Mosquito larvae must be in still water. If you agitate the surface of the water thoroughly at least once every 24 hours, you will kill them. Either splash it good, or let the tank run over every day. Make sure the water on the ground evaporates every day. Of course, fish are more interesting! YOu can get fish that are specifically for mosquitoes from some nurseries, or even your city might have them.
Wal-Mart has some tabs that you can drop in the water... says on the box it's safe for water troughs and ponds found in the garden center... Hmmm... never tried it... we only have two minis, so I have a smallish type large bucket that I change everyday.
Gold fish work in big tanks......dumping and scrubbing at least once a week works in smaller tanks.........a bubble bar from the fish tank dept at your local pet store works IF the horses will leave it alone
......a tad bit of garlic oil works as well it creates a barrier that kills the larvae.......ACV helps as it changes the acid levels in the water making it less hospitable.......
A small sprinkle of salt into puddles and standing water is supposed to kill them, so I suppose you could put a light sprinkle into the trough and it would work the same, yet it wouldn't be enough really to affect the taste of the water for the horses.
There is some sort of little fish(not goldfish!)that eats mosquito larvae; the city of Albuquerque was actually giving them away earlier this year! However, I don't know if they could survive in just a stock tank, once they ate all of the larvae-don't know what else they need to eat, beside larvae? I have one goldfish who has now been in a small stock tank-at first, a 100 gallon round, now, about a 75 gallon oval, both steel tanks. "Goldie" is now coming up on 5 years of age, and is the lone survivor of 3 who came with my flying squirrel when she was flown here from Houston, way back then! I put them into the stock tank; two were dead within a week or two, but this one has thrived! (I later found a home for the squirrel; just didn't have the time to devote to her!)
When I empty and scrub the tank, I just grab "Goldie" with my hands when I've scooped out enough water, and put him/her into a bucket about half full of fresh water, then "pour" water and fish back into the tank as it is partially refilled. I sprinkle in a bit of fish food the first day, but what the horses 'drool' into the water, plus any nasty 'skeeter larvae, feed him/her the rest of the time. Don't know if this would work with the larvae-eater fish, though.
The 'bubbler' idea would surely work, as it is very true that the larvae do not survive in moving, aerated water. I don't fill my troughs full, to discourage some VERY large resident ravens from sitting, drinking, washing/dropping their food(often parts of rabbits..
!)into them----URG!, so can't run them over-but use some of the other strategies mentioned. (After fighting those dirty ravens for several years, I now compromise by keeping a couple of shallow Fortex pans full of water out, well away from my water buckets and troughs. This provides a safe(too shallow to drown in)water source for the ravens AND all the other birds, like the mourning doves who are returning in some numbers after YEARS of being hunted out-hooray!-and keeps the ravens from soiling my other waters. I have seen them batheing in the pans! They are WELL aware that they are to use them, BTW-smartest birds around, but sometimes hard to have as "neighbors", as they will kill baby rabbits, and baby birds,too....)
Sorry-I drifted a bit off-topic....There are also those "mosquito rings", sold at garden stores. You might ask about them.
We got some things called mosquito dunks at the feed store, they are little round donut looking things, we broke one in half and put it in the tank. We haven't had any live mosquito larvae since. They are supposed to be good for a month. We just used 1/2 because it was made for a bigger tank than we have.
A lot of people here put goldfish in their fish tanks...but you don't need many. Also you should make some bat nest boxes to encourage bats to nest in your area...they eat hundreds of mosquitoes a night! Less skeetos...less larva.. you could also get some ducks! I've also heard of putting apple cider vinegar into the water (not loads) I do this and it seems to be working.