It could be the Nitrate (how high is it exactly?) and it could just be the stress of the cycle. It's not unheard of to loose a fish during a cycle, (it's actually weirder to not loose fish during a cycle.)
In order to keep everyone healthy though, you are going to need to take the goldfish
out and put it in a 20g tank. That's the minimum tank size requirement for 1 goldfish
. They are also cold water fish and do better without heaters, where mollies need the heat.
Have you noticed any white spots, fuzzy patches, clamped fins, bloating, pop eye, weird poops (ex: white and stringy), gasping at the surface or not eating from the other Molly? (Did the dead molly show any signs of illness or disease?)
*Also noticed this is posted in the "New to SALT WATER hobby" section - you might want to re-post the question in the Fresh water section to get more answers.* I would do it, but I'm not a moderator so I can't move the post.