04-15-2010, 12:46 PM
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I would be careful about purchasing corys in that tank.
First, obviously corys can't be foraging around the substrate 24 hours a day, they need rest too; and they are generally quite active at night. In my experience over 20 years with dozens of species of these interesting fish, some are out and about far more than others that I rarely see except at feeding times.
However, in a store tank, I would suspect this could be a sign of stress. Several factors come into play though. The length of time since they first arrived in the store, the water conditions in the tank, the number of corys and species are all relevant.
Corys do not travel well; they are highly susceptible to ammonia poisoning and when shipped too many in a bag they frequently arrive very stressed and at the point of death. I know of several store owners who regularly lose more corys than any other freshwater fish solely because of this. Often if they are not dead upon arrival, they succumb within a few days. Even moving a cory from one tank to another in my fish room can cause a temporary but significant change in behaviour, and my water parameters are near identical between my Amazonian tanks.
Most cory problems start with increased respiration--respirating faster than normal, especially if they are just sitting there--corys naturally respirate faster when swimming around and feeding. Second sign is lethargy, sitting motionless for long periods, plus increased respiration, is almost always a sing of trouble. In tanks with plants, they often do this closer to the surface, another sign of increasing problems. And sometimes they faint--just flop over on their side, still respirating--although this behaviour has been observed by me and others in perfectly healthy normal corys though very infrequently.
I can stand and observe a store tank of corys for several minutes and decide whether or not to risk buying them. I frankly don't really know what it is that I am seeing, but it is something. The few times I have taken the risk when I felt all was not well, the corys have died. I'm sorry if I can't really explain it.