Well I'm new to this but have read a lot on the net about tropical fish and know now that these 2 new fish I bought about two weeks ago are female guppies,the black mark on there underneath is what is called a gravid spot so I also know they are pregnant too. My problem is I already have 9 male guppies and although they all seem to be getting on fine this one male keeps picking on the biggest female. It looks to me like hes nipping her belly on her gravid spot, like hes trying to bite her and chasing her around the tank where ever she goes, I actually had to put him in a net for a couple of hours so she could get a bit of peace. He doesn't seem to bother her when the light is off but I like the light on now and again, no point having them if we can't see them eh? I have an assortment of other fish and they don't seem to be bothered by the new ones either, its just this one guppy. Any idea why this is? ???