You could just add Summit mosquito crumbles (
https://www.amazon.com/Summit-Responsible-Solutions-Mosquito-Bits/dp/B0001AUF8G/ref=sr_1_5) and no fish (easiest) so long as you do not have pets that drink from the container.
A fountain will probably be too big and splash outside the container. A spitter might work with a very low volume pump to add movement if you just want the sound of water, but it probably won't be enough to prevent mosquitoes since in a container that size you'd need to have a very low flow rate to prevent water splashing outside the pond.
As for fish, for a container that size, you could add a couple guppy (
Poecilia reticulata) or mosquito fish (
Gambusia holbrooki/
Gambusia affinis) if you also add filtration. Three male guppies would be OK since they are smaller (fancy guppies come in many wonderful bright colors which is why I use them in my small pond), or two female guppies, or three female minnows, or two male minnows since the male minnows are larger than the female minnows - DON'T add both genders as they could multiply and overstock your small container very quickly),
but not without sufficient filtration. I have no experience in minnows. I believe minnows are schooling fish and should always be kept in schools of three or more, but I'm not sure your container is large enough for three+ fish unless you add robust filtration. You could make it work with good filtration. I would also be sure to only buy one sex as breeding will be problematic in that small of a container: you could quickly get more fish than your container can support if they are allowed to breed.
For a container that small, you could probably just use an aquarium filter that's rated for twice the size of your container if you want to add 2-3 small fish (I'd size up since the filter will be doing more work in a pond-like container which gets sun than it would be doing in an aquarium which is cleaned more often and not exposed to sun, so you'll be having to clean the filter pads of algae quite often if the filter area is small). The Fluval U2 (
http://usa.hagen.com/Aquatic/Filtration/Internal/A470) would probably work as it has two filter pads and a biomedia canister plus a venturi valve to add aeration (here's a video that shows how to set it up to also aerate using the venturi value:
www.youtube.com/watch?v=K0b5JmJbXMc, and a video that shows you how you can tweak this filter to save money in the long run:
www.youtube.com/watch?v=Mz_HxKQ7Db4, and also a video about how the venturi system that adds the air to this filter system works:
www.youtube.com/watch?v=Cr6dPkrJWNo). Don't get the Fluval U1 as it won't be sufficient for an outdoor container. Once you add a good filter that adds oxygen plus filtration, fish would be OK to add (no more than three fish that get up to 3 inches, since 2 gallons per inch of fish is a safe rule of thumb for most small aquarium fish). Adding a mechanical filter to a pond means adding pond maintenance: you will need to remove your filter pads/sponges and rinse them regularly to maintain good filtration. Depending on how much sun and fish load, you might have to rinse the filter pads every week. The good thing is with the Fluval U2 filter, you need not worry about killing all your biofilter bacteria when you rinse your filter pads, since you can rinse the filter pads while leaving the biomedia canister in place in the filter to keep your beneficial bacteria in place. This means you can just pull the filter pads out and rinse them in the sink in tap water (or use the garden hose), then treat them with a little water conditioner before adding them back to the filter in the container. Guppies do well in moving water, so would be fine with this filter (they love swimming into the current). I am not sure how minnows do in water with a lot of movement. Others may have other filtration ideas. I don't think I have ever seen a pond filter for something that small though, so you'll likely have to use an aquarium filter. Most aquarium filters will say "indoor use only" but should be fine so long as the plug stays dry (I use an aquarium heater all winter outside for my pond with just the plug up off the ground under a table to keep the snow/rain/groundwater off it).
