A few weeks ago our air pump that had two lines in to our 150l tank died with only one airline now working. We have ordered another one, arriving Wednesday but we've noticed our older corydory's are dashing up to the surface and back down again and swimming around the whole tank where as the younger ones are happily stuck at the bottom.
does anyone know the reason for this. It's very disconcerting to look in the tank and see a cordydory shoot past your eyes at the top.
All water parameters are ok slight nitrate increase but that will disappear when I do the water change and cleaning later
This is completely normal behaviour for corydoras. It used to startle my other fish when mine did that, especially as there were a few collisions when an unsuspecting fish was in the way.
I don't know why the younger ones aren't doing it, or why the older ones have only started doing it, but I can only assume that it's one of those aspects of behaviour that changes in frequency at different stages of the fish's life and/or that the failure of the airline drew your attention to it more. Unless it's happening constantly (ie the same fish dashing up vertically repeatedly), then you have nothing to worry about; if it is happening constantly, then the replacement airline once installed ought to solve any increase in such darting-up behaviour (eg due to any reduced oxygen levels) back to normal activity and darting-up levels.