Yet another aquarium

Our hardware store (the houseware department) has a little assortment of aquariums. Quite odd for a hardware store, isn't it? Somebody working there must have a special interest in aquariums, because they need continuous maintenance. Here I blipped the biggest one.

This is very small and holds perhaps 2 liters of water. And one fish, which is very attentive to the environment outside its little water world. A light knock on the wall, and it immediately took cover behind the plant.