Hi, I've been building up my coldwater aquarium over the oast few weeks with all female platies but noticed a tiny fish in the tank earlier this week. Could one of them already been pregnant and dies this look like a baby Platy?
Young fish at that age tend to all look somewhat alike, but if all you have in there are plates then it's a baby platy. It's very often the case that female platies are pregnant when purchased - presumably, this one is the only survivor as they usually produce 20 or more at a time. The female could produce more in another month as they can store sperm and release it at intervals to fertilise more eggs.