Hello, I have a mystery on my hands here. This is the account of events:

due to some ironic circumstances, at work i have a 5 gallon round tank in which I had three glo danios, two black mollies, and a bristlenose pygmy pleco. I know that that size tank for mollies is small, and i actually have ordered a special tank (10 gallons) at the pet store, one that has lots of space, but can be kept at work. Its really cool looking.

My fish have lived in my current tank for about two months now. Last thursday, I did a 1/5 water change. Everything seemed fine on friday, and i went home for the weekend. Now, when i came back to the office monday, i saw that BOTH mollies were dead. They looked like they'd been dead for a while. All the danios, and the pleco are just fine. The tank water looks clean, PH and ammonia levels low, and the temperature is 78 degrees. My first thought was, well maybe it was a bad batch of mollies, but i got both fish at a different store! I know that mollies like kind of brackish water, and didn't have any aquarium salt in there, but i do use a water treatment that claims to make water safe for all freshwater tropical fish. PLUS i didn't want to put salt in the tank on account of my pleco. I just don't get what killed them, when all the other fish seem fine. Im still kinda new to all of this, but i was basically wondering if my other fish were safe, and or how to proceed.