My old 2x1x1 tank is a prefab plastic job, with integrated heater filter/pump and lights. It now gets used to raise fry.
The heater and filter/pump are enclosed behind a plastic partition slotted in at one end of the tank. Water passes through a grill in this partition, via 3mm slots, past the heater, up through the filter, then pumped back out to the water surface.
This morning I pulled out the pump/filter to clean. I discovered what was obviously Cory eggs down one side of the canister - they were white, and so non-viable.
I went for a torch, and shone it down behind the partition. Sure enough, One mature female Corydoras, of a good 3cm.
The young lass must have found her way behind there at about 3 or 4 weeks old (thru 3mm slots), and been living a hermits existance ever since! The others were probably passing her scraps of food.
(If you look real close you can see groups of four vertical strokes, intersected with one horizontal stroke, marked on the walls.)
