I don't think it has any bearing whatsoever that the aquarist is a teacher. This person is running a tank with 2.5 gallons per fish. That's better than what others here do. I would never barge in out of the blue and criticize that person.
If a fellow fishkeeper asks for help, I'll offer it. But "ask" is essential. They open the discussion. I haven't gone out to educate a store worker or fellow fishkeeper since I was a teenager and knew everything. I wish I had been smarter then.
3 danios. Small group, but a common number. We have no idea of tank dimensions (which my personal rule says should be 8 to 10 times the total adult length of the fish). We don't know what kind of filter there is. How does the water circulate with the kit they've used? We don't know if the OP is familiar with keeping zebra danios, or has seriously looked into their needs.
I was a teacher, by the way, with a 75 gallon classroom tank that first overwintered a couple of shubunkins from a school pond, and after a summertime heron ate them, housed a community of threatened Goodeids, which I bred and distributed. I had kids who'd watched a youtube video tell me water changes were dumb. I was told I needed a large predator to eat the excess babies, because they were ugly and that would be natural. I was told I fed them wrongly. I was told to raise the temperature to 27, because that's what so and so online said for Discus. It was always sincere. I always said I'd think about it. The triumphant experts would wander off and lecture someone else on something else, and I'd run my tanks and hope there wouldn't be a next one anytime soon. Fishstore workers, importers, aquarium writers, speakers at clubs - they all experience this. It's not fun for them, and is rarely constructive.
