So I just finished cycling my first 10G tank. I cycled it with 2 mollies and a goldfish. Don't ask about the goldfish....it was for my daughter. Yesterday I came home to one of my mollies dead. Weird I thought. Now my remaining black molly is spending most of it's time in hiding.... Should I be worried he's sick too or is he likely just lonely and stressed and needs time since losing his friend (could be a female....I don't know how to tell!)? I haven't seen any agressive behaviour from the gold fish, but the gold fish is huge compared to the molly.... What should I do, if anything? Ammonia and nitrite are nil, nitrate is a little high.

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.

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These are fish that was used to cycle with, they are probably dieing from the damage that can be done using fish to cycle with.

The goldfish and the molly shouldn't be in the same tank as they both have different requirements. Goldfish being a coldwater fish and the molly is a tropical which needs warmer temps than what the goldfish should have.

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Is the tank heated?

The fish may have had gill damage or other effects of the cycling that they could not recover from.