If the scales aren't sticking out - they say it's like a pineapple if you can imagine that (I've seen other fish with it, and confirm it looks a bit like that!) - then it may not be dropsy. It's pretty noticeable when it is, but the downside is that it's difficult to diagnose until the pineappling happens at which point it's too late to do anything about it. If it is dropsy then there's a theory it's to do with poor water conditions/unclean tanks, but I've seen it develop in perfectly clean water before in fish like mollies so it could be to do with poor quality breeding and/or a problem existing in the LFS/farm before you get them home.
When you feed have you kept an eye on what he/she does? Is you guppy 'hogging' all the food? Could be that if so. If not, I'm really not sure: I have no experience with fert dosing, so I wouldn't know if that's a possibility. Maybe the folks in the planted tank area might have an idea if there's a risk?
Hope you can get it sorted.