The "must-haves" for every coral keeper?

[Madmak]

A good coral glue (any cyanoacrylate gel like Krazy Glu) and some live rock rubble means you are always ready to frag something when you need to.

A very good little flashlight allows you to check on things when the lights are out or look in any live rock holes, that's where the bad worms and crabs always are.

Obviously a good test kit that you trust and, more importantly, know how to use and read. Water parameters are always where I start troubleshooting a problem.