I have no idea why they are hanging out at the top, but I have noticed that the bigger fish in my 150g are always swimming in the top half of the tank, the smaller ones stay down towards the bottom and in the rocks (except at feeding time)... Maybe its just an african thing?
I agree that you need to get your nitrites figured out. You've either tested wrong, added too many fish at once recently, or have some issues in your bio-filter that need straightened out. Big water change is definitely a good idea
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