A few things:
I think the best path is to break down the tank and start over, properly, before you do anything else.
- A 5 gallon tank for a newbie in the hobby is not a good choice. Chemistry swings are very large in small tanks. When you combine that with your lack of experience, it won't end well. I know, I once tried getting into the hobby with a 5 gallon tank. It was a colossal failure.
- You need to make your own salt water using RODI filtered water. DO NOT buy your water from the local fish store as they give you the OLD water and replenish their system with new water.
- Your tank was too immature to add corals; a tank needs time to develop its ecosystem before it can support delicate creatures.
- It is essential that you provide enough light to the tank. While the tank maybe well lit to the human eye, your need good spread and penetration as well.
I think the best path is to break down the tank and start over, properly, before you do anything else.