08-03-2011, 11:48 AM
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You were witnessing normal spawning behaviour. Fish generally spawn in the very early morning, just after dawn in the wild. And Cyprinids and characins (your Danio are Cyprinids) spawn by having the males actively pursue one or more females, attempting to drive them into plant thickets where the eggs will be released and fertilized, in the method known as "scattering." The sticky eggs fall among fine-leaf plants and hopefully survive predation.
This behaviour is stronger in some species than others. Danio, being very active swimmers, are more active drivers when spawning. This is why it is often suggested that there be more females than males. With more males, the females will be pursued relentlessly and often to the point of death. In an aquarium they have no escape, unlike the wild where they can swim away and "hide" among plants, rocks, wood, etc. And there are enough of them that the males pursue other females. In the confines of an aquarium, more females is preferred.
I recently witnessed this behaviour with my Emperor Tetra, the males of which are also very aggressive in spawning.
This is another reason why tank sizes are given for our fish in the profiles; some species need more space than others, even just for spawning.