I had a 1/2 whisky barrel pond, but with a liner. Just a few plants
in it. It got really skanky smelling until I put a small aquarium air
stone in it powered by a small battery powered air pump. Cleaned it
right up. Don't know if it was the added oxygen or just the
circulation.

When low oxygen water at the bottom of the barrel is circulated, it
circulates up to the top, where it oxygenates. A technicality: the
water gets more out of being circulated by the airstone than it does
from the air bubbles. What you need to know: yes, the water is
oxygenated.

I have some little wriggly things in my regular pond that are not
mosquitos. A larger head and not as black. Some kind of gnat larvae.
If I go out at night and shine a flashlight into the water they
approach it and within a few seconds come to the surface and
metamorphose into a little flying bug and take off. It's bizarre to
see.

Rosey red minnows thrive well in small containers like this, and will
eat various insect larvae with glee.

C//