Looking at the damage to the side of the glass, I think it's indeed an old tank. But maybe they did some sort of cleaning and added in new (non-acclimated) water
@Mr Gharial There was no recent major cleaning done on that tank, there's enough visible proof of that - note the algae between the substrate and glass, the unkempt appearance of the grating, and the notable growth of algae in various areas, besides the very obvious waterline buildup. Also it's a bit unclear from the picture set but this tank appears to be the middle of a connected set of tanks - one that would appear to be difficult to do a water change on without affecting the others. Numerous fish are visible in other photos of the lower tank, so I'm not inclined to think non-acclimated water is the reason here.
@Great Argus You're correct, it's the middle of three tanks that share water. The first one has gar and catfish, among a few others. No clue what the species was in the third tank, some black fish.
