There are a lot of non-fishy things which can be used in aquaria. I often wonder about plastic and other artificial flowers and plants. The "silk" flowers I would avoid if they really are silk or have a scent. Also, avoid anything distinctly organic. Some other plastics leach toxic "plasticizers" and we need to always ask if they are "food quality" plastic. The plastic plants from aquarium suppliers should always be food quality, but you are well advised in looking to see if something is on them. (Bleach garage sale aquarium plants.)

Some otherwise safe plastic plants may be in a garden center where they have assumed the scent and a residue of the Ortho plant fertilizers and herbicides. Those babies need to be avoided or if purchased, really soaked and rinsed. I might even bleach them.

Of course, it is possible that your guppies were ill for other reasons. Has the tank cycled (is it over 9 weeks old?) What temperature is that aquarium? Were they behind in partial water changes? When you "clean" the filter, if any, do you still leave it a little discolored (discoloured) so that some of the "good" bacteria are still there? Could the fish be suffering from something which rode in with them from the shop? Are there little worms extending from their vents?

Your evidence, by way of the partial water change, suggests that there might have been something on the plastic. And that is sagacious, suggesting that we be selective when purchasing plastic plants and in cleaning them off well.

Keep up the partial water changes with seasoned water. If there is any reason to doubt the plastic plants, get them out. With a little TLC, your male has a pretty good chance of making it.

If he seems ill still, let's look to what might be ailing him. If there are no spots, dots, hitch-hikers, frayed fins or discoloration on him and you have been consistent with the partial water changes, consider the possibility of internal parasites. (Gee whiz, that's what he always says...)