I worked out in the end that it was feeding them the brine shrimp wot done it... I think so anyway.
We had a couple of angels that laid eggs. Guarded them for a few days then ate them :(
I'd be reluctant to stock up to capacity on platys. They must breed relatively easily. I remember getting up one morning to a tank full of little tiddlers (they were soooooo cute though). Might have been swordtails come to think of it.