King Archer

There are actually a handful of archers that can stay permanently in freshwater. Sometimes this is a characteristic of the species, other times you might find certain populations (e.g: Toxotes chatareus) are permanently freshwater.

What size are these fish?

Going by patterning alone, these look like T. jaculatrix. The dark bars by the dorsal fin extend from the scales into the fin itself, which can occur in maybe one other species but this looks nothing like the smallscale.

 

Many archerfish that frequent brackish waters are quite tolerant of fresh water, and may move some way into freshwater river systems; though you are right in saying on a temporary basis. However, there are a few landlocked populations of otherwise brackish archerfish species; at least with T. chatareus that I know of. If I'm not mistaken, an Australian hatchery spawned these in captivity not too long ago.