I think another contributor to this is the fact that you changed your gravel. This removed most of your bacteria which caused the mini-cycle.
Also, plain gravel is completely void of any nutrients. And depending on the size it doesn't hold nutrients around the roots very well. Try adding an aquatic soil like Black Fluorite or Eco-Complete (they're black like your gravel so you won't have to mess with changing it out again, just throw it on top).
Don't get discouraged. There aren't very many people who get it right the first time.