freshwater help

I have a single Molly and a neon tetra living in my 10 gallon tank. One Molly died and my other 5 tetras died. I have test strips and nitrite and nitrate are high. The kit says to do a water change so did 30% one but there's no change. I would buy more neons but I don't want to risk anymore...

So I’ve done a lot of googling, and everything I can find is all about acclimating freshwater mollies to salt, but I can’t find anything about the opposite. I was setting up my saltwater aquarium and wanted some cheap cyclers for my tank, so I decided to go with mollies because I read that...