Trouble With Mollies

My 10 gallon has a couple Dalmation Mollies, two Gold Dust Mollies, one Columbian Tetra and a Cory. I've only had the Dalmations for 3 weeks. A few days ago one of them died. Today, I see the other Dalmation is hanging out near the top of the tank more nose inverted (tail lower than head). Today's readings are ammonia-0, nitrite-0, nitrate-40, Ph-7.6. I syphoned gravel (5g).

Why are these Mollies struggling? The other guys seem fine. I really like Molly behavior but they don't seem to like my tank. I read that they like salt, is this the wrong setup for them?

 

Hmm... what are you feeding them?

 

What is your water temp?

 

Your Cory might get lonely, but not much you can do in a 5 gallon...

 

A 10 gallon is a bit small fir mollies they prefer more room and have a pretty big bioload

 

And how about water circulation? What are you using?