If this is the tank that's only been around ~2 months you'll come to see how saltwater tanks can be. Basically you're attempting to make a full ecosystem from the bacterial level thru to fish and inverts. This amount of time is very small and changes in water make-up and quality WILL happen. It's a matter of having the tools and mixture of animals to minimize these changes. Even the most established tanks can have problems so don't get discouraged.
Basically you're just going to want to check all your parameters to get an idea for what has happened so you can fix it in future. Most of the advice on saltwater tanks is anecdotal at best so remember that.
