Did I already answer this? Can't remember and can't be bothered to look. So here it is: A 300g would be a Wyoming native tank: Lots of rock work, few or no plants, possibly a land area above with native mosses, sedges, waterfalls. An industrial strength chiller to keep the temps in the 45-60F range, gigantic filter and power heads for mountain creek-level current.
Stocking would include:
Yellowstone cutthroat trout
Plains Killifish
Mottled Sculpin
A school of silver minnows--there are a ton of different species and they all pretty much look the same.
Maybe some long nose dace: Not the greatest looking fish, but great personalities.
If I did a Paludarium with a land area:
a herd of boreal chorus frogs
or, if the land area turned out being more of a desert enviro, some sagebrush lizards.
There, wouldn't that be a lovely tank? It's so fun to dream...