plump danio being chased by two
 

Yesterday I bought 3 zebra danios and this morning decided to add 2 more, they're all doing fine not until I've noticed later today that 2 danios are chasing the two plump danios without a pause. After searching the net for some possible answers, this is ym conclusion: i have 2 males (very prominent yellow between the stripe) and 3 females. The 2 females I believe have eggs because they are rounded and the two males kept on chasing them in cricles and they don't mind the other possible female swim around the tank. If everything is correct, within the next few days I will be expecting eggs in my tank but I dont have any plans of breeding them, so if the possible babies will survive, then good. By the way the y are in a community tank with 5 harlequin rasboras, 3 otos and 3 cory cats.

You may well be correct; an easy fish to spawn but fry surviving is highly unlikely as this fish like most cyprinids and characins will very readily devour the eggs immediately. You can find more info on this fish in our profiles, second tab from the left in the blue bar across the top of the screen, or click on the fish name if it is shaded in posts, example Zebra Danio.

another observed behavior but not of danios but my harlequin rasboras, one with a big rounded belly and the other rasbora, they kept on swimming together, swimming upside down under the plants leavessomething like scratching their bellies on it. another courting fish in my tank?

Yes, that is spawning behaviour of the species. The male would have wrapped his tail around the female under the leaf. Again, the eggs will be almost immediately eaten once spawning is completed. Sometimes the fish sort of "practice" so it is possible to see the behaviour without eggs being actually laid, but it is a sign they are nearing condition for actual spawning.

fun to watch these behaviors