What do you mean by Brackish? The term can cover a very broad range of salinity.
I keep my 700gal pond at between 3 and 5 parts per thousand of salt. If we get a lot of rain it might drop to 1 or 2 ppt, but I've got it stocked with Australian native fish that can tolerate a broad range of salinity. The pond works well and is very easy to maintain.
I agree brackish is just going to fail miserably, Have you ever tried running it fresh water year round ? That alone can be tricky in a small pond the size your talking about.
Every time it rains or snow melts the water parameters are going to to be all over the place. I also suspect that some serious filtration issues will occur at sub zero temperatures. When it does get cold enough it'll still freeze over, the ice layer is fresh water and the salinity below will rise.
Brackish ponds/lagoons exist in nature but they are not contained bodies of water. Its occurring in zones where the fresh water is flowing into the ocean.Even in northern climates those Fish are never trapped in frozen over conditions with no running water because they can migrate to sea /remain in the tide zone that keeps the water moving and ice washed away. It just isn't something you can mimic or keep stable.
15 parts per million is negligible. That's basically fresh with a touch of salt. If you meant 150 ppm, or 1.5 parts per thousand, well that's still no problem. If you meant 15 parts per thousand, that's close to seawater and might be more of a problem.
Anyway, here's a clip of my pond. Running at 4 parts per thousand. These fish don't like the cold though - I'm in the tropics in Australia.
I have seen a salt water pond in the California desert (with a ready supply of replenishing freshwater off course). A brackish pond should be no problem, especially if it is closer to 1.005sg. However, im not sure what brackish animals tolerate that cold weather. Your best bet would be truly euryhaline fish and overwinter the pond with a deicer or greenhouse. It could be done but i have no idea where you are going to get brackish cold water stock, there is barely any tropical brackish fish to choose from as it is.
I raise brine shrimp in outdoor ponds in Wisconsin in spring, summer and fall, and even though the salinity is high, the water separates and fresh water freezes in about 3/4 of the pond space, leaving a shallow dense brine thawed area in the bottom few inches.
And I agree about the fish disappearing, I also have cichlids in a larger pond during the warm months, and most of the time, they are nearly invisible due to their camouflage. Koi and goldfish are bred to be seen from above, natural fish coloration helps them disappear.
There are a couple dozen cichlids in the pond below.
They are only seen when they move into the shallows.
Then the problem becomes predation by raccoons.
