Our tank has been cycled since the begining of April. Yesterday our female Molly died and this morning I woke up and the male is dead. I tested all levels yesterday and today and they are were they should be. 20% water changes are conducted weekly and the temp is at 78.

We had a male and female silver molly die late April and replaced them with dalmation mollies around mid May. All the other fish are doing fine (redtail, albino cat, male dwarf gourami, female gold dust molly, 2 white hifin tetras). All of these fish have been in the tank since Jan.

What could I be missing?

Strange! I was waiting to take the male out until my son woke up so he could decide to bury or flush. He's alive. He was on his side on the bottom this morning and appeared to not be breathing. Now he will swim for a bit then go back to the bottom. He's not doing well at all.

Do you use aquarium salt in your tank? Mollies are brackish water fish and thus need a little salt in the water in order to thrive. They'll do ok for awhile in completely fresh or completely salt, but it could cause problems a few months down the line.

hi
hope ur using aqua salt and not table salt? (just thought i'd ask)
the mollies could be from a bad batch of fish , we al get these problems with fish dieing mostly because of the fish health and where u bought them from its dosnt sound like its anything to do with your water quality

my tank and inhabitants

Not only do mollies prefer slightly brackish water, but they also prefer hard, alkaline water. I've heard that they are prone to disease when those two conditions are not met. If it's only mollies that you are having trouble with, probably best to just avoid them. There are a couple types of fish that i haven't had luck with dispite being successful otherwise. You could try platies instead. Best of luck.