You lower salinity with a water change. However, this will have a negative impact on your corals and possibly crab. We need to know EXACTLY what you have living in this aquarium. Every species of invert, coral, fish, crab, snail, etc, etc. What is in this tank?
At this point i hope you have picked up a garlic supplement. If you have, then at this point I suspect you are already seeing an improvement.
By the way, I am glad to see Jack Middletown posting the advice given above. The approach of setting up a hospital tank and adding copper based medications is a very traditional approach to ich treatments. You will find a ton of literature recommending this treatment. Unfortunately, it is an approach that I personally disagree with based on my personal experiences and conversations with hundreds of people on these internet forums over the last decade. There have been many recent published articles that agree with me. You would need to reference publications written in 2008 or 2009, such as FAMA and TFH, to support my point of view.
Keep in mind, treating for ich is not an exact science. There is not really a "right" answer here. The hobby is still studying these techniques. Major universities and still "discovering" how ich is best treated. I am just giving you my personal approach.