Vegetable oil does not work well because it beads up on top of the water and does not spread across the surface.
It also goes rancid.
Mosquito larva breathe air through a little tube while they float on the surface, so "oil" remedies are designed to cut off their air supply.
You can try a horticultural spray oil .
It only takes a few drops to form a microscopically thin sheen across the water.
This is recommended for bird baths so it's pretty safe for the innocent bystanders in your yard.
You can also try mineral oil .
It's completely non-toxic.
There are also non-chemical larvicides (that are mostly mineral oil) which are made specifically for this situation.
These pet-safe mosquito dunks contain Bacillus thuringiensis ( Bti bacteria ) which mosquito larva eat and die
but it is completely harmless to everything else: pets, birds, wildlife, etc.
If your pond water is otherwise healthy, you can always try raising some fish.
Goldfish, koi or, yes, even the aptly-named " mosquitofish " love the stuff.
gardening.stackexchange.com/users/85/Robert Cartaino
You don't have fish living in it do you?
If not, I would drop some pool chlorine in there.
gardening.stackexchange.com/users/336/myron-semack
I have found only one solution to mosquito larvae.
Drain your fountain once.
Use a shop vac to remove leaves, debris and dirt.
Let it dry in the sun for one day (if you are patient).
On day two, refill with water , and place a jacuzzi chlorine float in the basin .
Close the chlorination vents so your chlorine is not dispensed too fast.
Chlorine tablets are the size of wine bottle corks (available at big box home improvement store, or pool supply).
Set your fountain timer to operate at least a few hours a day to cycle the water (I run ours from noon to 10pm for ambiance).
Once a month, top off the chlorine float.
You will never have mosquito larvae again.
I keep my chlorine levels high enough where you can smell the chlorine on your wet hand.
I would rather have a sparkling clean fountain and replace my pump and parts once every five years, rather than change my water once a month because of larvae.
gardening.stackexchange.com/users/2470/Rupert
Probably the only way that you'll combat the larvae without adding anything to the water ( mosquito eating fish , those donuts ) is by draining and refilling the fountain on a regular basis.
Unfortunately, most of the recommendations I've heard speak of daily changing (can't find any references at the moment).
The aquatic life cycle of mosquitoes is 5-14 days, so you might be able to get away with changing the water every few days.
I've also heard recommendations of aerators, waterfalls, or anything else that keeps the water moving
(I'm assuming your fountain has some kind of feature like that already?),
but I'd guess it would need to be fairly aggressive in the amount of water it churns to minimize mosquitoes and their larvae.
I'm guessing it's going to take you a bit of trial and error to figure out what minimizes the both mosquitoes and the inconvenience of having to battle them.
You may even want to just let it sit dry during the worst times of the season.
And make sure you and your neighbors minimize any other sources of standing water around your yards!
gardening.stackexchange.com/users/93/rsgoheen
I think fish would be a good solution, but not goldfish .
They are nasty, producing lots of feces.
If you put a Plecostomus in there, it would eat the algae, and I have read that zebra striped Danios will ferociously eat mosquito larvae.
I am looking for solutions before I put in my water feature
, I'm no expert.
I think the oil would just look gross.
Hats off!
gardening.stackexchange.com/users/2117/Cherre' Holley
I water my plants with fountain water.
Then I use chlorinated water to refill the fountain.
The chlorine kills algae and mosquito larvae.
The running fountain removes the chlorine.
Repeat a day or two later.
gardening.stackexchange.com/users/13832/PanamaVet
Can you drain the fountain?
If yes, then drain the fountain and pour antiseptic on the fountain.
I cannot think of other method.
I won't put a fish into fountain.
gardening.stackexchange.com/users/140/lamwaiman1988
I found lots of mosquito larvae in my fountain. How can I get rid of them?  A local store has the doughnut thingies but they are expensive. I'd prefer a homemade solution if possible.  I tried vegetable oil. It worked but looks ugly.  Any other suggestions?
How do I keep mosquitos out of my fountain?
gardening.stackexchange.com
<pests><mosquito><fountain>
gardening.stackexchange.com/questions/1039
gardening.stackexchange.com/users/384/Kevin Lawrence
Vegetable oil doesn't spread across the surface. You can buy horticultural oil sprays and non-chemical larvicides. You could also try an aerator to keep the water moving. Mosquito larva breathe air through a little tube while they float on the surface, so oil cuts off their air supply. Vegetable oil does not work well because it beads up on top of the water and does not spread across the surface, but you can try a  horticultural spray oil . There are also  non-chemical larvicides specifically for this.  If your pond water is healthy, you can always try raising some fish, but not goldfish. Probably the only way to really combat the larvae without adding anything to the water is by draining and refilling on a regular basis. The aquatic life cycle of mosquitoes is  5-14  days, so you should change the water every few days. You could also try aerators, waterfalls, or anything else that keeps the water moving.
Mosquito larva breathe air through a little tube while they float on the surface, so oil cuts off their air supply. Vegetable oil does not work well because it beads up on top of the water and does not spread across the surface, but you can try a  horticultural spray oil . There are also  non-chemical larvicides specifically for this.   If your pond water is healthy, you can always try raising some fish, but not goldfish. Probably the only way to really combat the larvae without adding anything to the water is by draining and refilling on a regular basis. The aquatic life cycle of mosquitoes is  5-14  days, so you should change the water every few days. You could also try aerators, waterfalls, or anything else that keeps the water moving