I have a 10 gallon tank and started off with 3 female Platies. A few days ago I added 1 more male Platy (I was advised that there needs to be more females than males). Everything was fine to start with and he settled in straight away. All of the fish were getting on well with each other and seemed to be enjoying their new surroundings. However, I noticed today that one of the female Platies was pretty much hiding from everybody else and not looking as active as normal. I also noticed that its main fin (dorsal?) was not as upright as it is normaly. Having watched the tank for a while I then began to notice that the male Platy keeps chasing all of the other fish and keeps prodding them especially the one that is less active. Is this normal behaviour? Should I be worried? Any thoughts would be greatly appreciated!!