How to manage a smallish water feature in a countryside garden.

I live very near the National Forest, at about 700-meter elevation in Nikko, Japan (36.748240, 139.566356). It's a modern house in what is clearly a small housing development. I have a very old water feature on the property that I'd like to use to my best advantage. The water basin is 75cm long by 36cm wide and 23cm deep; the walls are about 2 to 3 cm thick.  I moved here in January and since about late March it's been at least half-full of water. Now in late July, it's completely full. It's made of stone and is old enough to have plenty of moss and other plants that like water attached to the exterior. Lot's of leafy debris is decomposing in the bottom of the vessel.

I thought about making it a frog pond, but I think it's too small.
I thought about buying a few hardy koi-like fish for it (I'll probably have to have them winter over in the house), but I don't know if that's doable.
There seem to be a lot -- a LOT -- of dragonflies flitting about now, so perhaps it's a natural dragonfly hatchery.
But for sure, it's a mosquito hatchery and I'd really like to find a natural way to stop that.

I can probably get a small battery-operated bubbler that would disturb the surface enough to prevent the mozzie eggs from hatching, but it will probably prevent the dragonfly eggs from hatching too. I hesitate to put something in the water (vinegar? baking soda? cooking oil?) to prevent the critters from laying eggs, but that sounds extreme and may harm the water feature.  A professional garden architect has said it's very old and doesn't want to move it. It sits under a leafy Japanese maple now, but that tree will be moved to a better location probably by the end of the year. In other words, the feature remains in the same spot, but it will go from heavy shade to full sun in 2022.

Any comments or suggestions? I really don't want to drill a drain hole or make any other drastic changes.  Hope you can see the photo.

Hi Barbara,
That is beautiful water feature. If it was smaller, I might suggest emptying it every few days to avoid mosquito larvae, but it is just big enough that that would be big task.

I don't like the idea of poison in the water either, though putting something like BT in, would probably be effective.

Given the restrictions, my two suggestions would be to encourage the dragonflies and frogs in the area to use the water. Perhaps some ramps or steps up to it would encourage frogs, toads, and other amphibians to help eat the mosquitos or add some sort of minnow. (Maybe both, but someone with more experience would have to let you know) If the minnows don't survive the winter, they can be fertilizer for the garden and are not too expensive to replace in the spring. If they do survive great.

**edited for clarity

I would not add any type of carp they all get to big, but some small fish might manage, or you could cover it in a net and turn it into a mosquito trap  
https://permies.com/wiki/144203/pep-homesteading/PEP-BB-homesteading-straw-mosquitotrap like the ideas here.

Cool little pond! You could put a couple of mosquito fish. They don’t get very big. Around 2-3 inches

Also agree with making some sort of ramp / stair so different frogs and stuff can get in and out.

Thanks, everyone!   I like the idea of ramps for the frogs and mosquito fish to eat the eggs.  I should put a ramp in it as well so the frogs can get out too.  I also read that article on the mosquito trap, but I'd like it better if the toads would eat them.  I haven't heard any frogs, so it may be too dry for them here.

Except to drain it for the winter, I don't think there is much to do right now, except read up on frogs and toads, and mosquito fish.  I only have a kind of a "modern-day" home centre and while they have a large pet centre, I doubt if they are stocking mosquito fish. If any of you have some resources of Permies links, please post them here. I promise to read them and post the results.  Cheers! Barbara