Originally Posted by joshcmayer
Also, I'm not quite sure what cycling is other than it's a long process. could it help in this situation? Sorry for my inexperience!
Cycling a tank is when you establish a healthy bacterial community in a previously barren tank. So when you start from scratch in a new tank, it takes a while for the microbial community to balance out into a nice, stable, fish-friendly form.
How long was your Molly in the tank & doing well before you added the guppies?
My first thought is ammonia / nitrite poisoning from the increased fish population. I would immediately do a 50% water change --- even if it turns out to be something else it'll give your fish some healthy water, so there's no downside.
Do you have a kit to test the water chemistry of the tank? Does the tank water look a little cloudy/white? If you don't have a water chemistry kit or the water looks cloudy I would try daily 50% water changes for at least a week and see if your fish become more active.
A cloudy tank can indicate a microbial bloom. Water changes will help with this also. Your tank might just need to adjust to the increased "bioload", or amount of waste products coming from your fish that your microbial community has to break down into non-toxic forms.
How long is each fish? 5 gallons is definitely on the small side, the general rule is 1" of fish per gallon. Do you have any plants?
Hopefully some of this was helpful! Best of luck, I hope your Molly perks up