Molly fish just suddenly died?

We have a 10 gal tank with 4 fish 1 in a molly. Most energetic fish we had. We had a bubble maker in there and finally got it hooked up last night since we had forgotten to grab the hose (herpderderp) . This morning I got up to find the molly laying on the bottom of the tank and any time she would try to swim up it was like she just didn't have the energy and was straining. Then within 2 hours was just dead. One other fish is starting to act pretty mellow now too, but the little guppy we have is as happy as can be. What's wrong with our tank? We've always kept the water well filtered and conditioned along with a ph balancer and an automatic no adjusting censored heater.