Do yourself a huge favor right from the start. Replace the coral with sand and remove the undergravel filter. They do not do well with saltwater aquariums or freshwater in my opinion.
You cannot use undergravel filters with sand either so both should be removed.
No need to take out your water you have in there right now, scoop out the crushed coral and filter and put your sand in.
A good HOB (hang on back ) filter will work well. A power head or two will keep a nice even flow in the tank too.
When you do this and your sand is all in add some live rock. You don't need to add all the rock you want at once. This hobby is expensive so taking it slow is your best bet.
Another piece of advice, do things right the first time. It will save you alot of money and heartache in the long run......
Also, using tap water will cause you trouble too. RO or RO/DI is best. You can search this on the forum and finds lots of feed back about this.
Ask lots of question, this is the best forum for all the help you will need to succeed in this hobby.
Good luck and welcome to the board