One of my two month old guppy fry has turned listless and sort of gasping at the tank surface near the filter. I'm doing a gravel vacuum tomorrow and do them twice a week so it can't be ammonia. She is about a half inch long and will still eat but isn't growing terribly fast or coloring up like her sisters.The others are fine. I don't want to cull her. I was thinking a lack of oxygen? There are no missing fins or any blood anywhere.