After having had to deal with Ich myself, and having done a lot of reading around, I found a lot of information (and misinformation) was needlessly overcomplicated.  I decided I would try to write a condensed version, which gets to the point, and explains what you need to do to cure and prevent this disease.

This is one of the biggest killers of marine fish in the home aquarium, so it’s important to understand that it can be relatively easily cured and even better, prevented. 

Myths
I’m going to start with dispelling some myths, as I don’t want anyone to think I’m babbling on about some complex cure, when they could just throw in a bottle of magic potion, or a cleaner wrasse and it will all go away…it won’t.

• There are NO REEF-SAFE CURES.  I can’t stress this enough.  People that make chemical cures and people that sell chemical cures claim a lot of things that just are not scientifically proven.  Read around any trusted marine biologist website/books and you’ll find them saying the same thing.  Either these ‘cures’ don’t work, or they will harm other life (including the micro fauna that makes up your biological filter and ecosystem).  Don’t do it!
• Cleaner fish, shrimps, etc. can pick off the parasite.  Nope!  It’s buried under the fish’s skin.
• Dips will get rid of it.  Nope!  They might give some relief to the fish, but again…it’s buried under the skin.

What is it?
It’s a parasite.  It has various stages it goes through in a lifecycle.  We can only see it with the naked eye, in one of those stages, when attached to a fish.  These are the white spots that give Cryptocaryon Irritans its nickname.

These are the 4 stages of the lifecycle:

Image from ATJ's Marine Aquarium Site - Home - Welcome

How do I get rid of it?
There are two things to deal with here.  Your fish, and everything else.  Here’s the bad news.  You need another tank.  Nope, there is no other way.  You need another tank, or one of two things will happen 1) your fish will die 2) you will try to treat the fish in the display tank (DT) and end up harming/killing your inverts, thus destroying your ecosystem.
I’ll explain why as we discuss how to treat your fish and everything else.

How to treat your fish
There are only two proven ways to do this.  Hyposalinity or Copper treatment.  For both of these you need to set up a hospital tank.  This is very simple.  You need:

• A tank big enough to house your fish
• An appropriately sized heater
• A simple sponge filter
• Optionally, some PVC pipe for the fish to hide in/behind
• NOTHING ELSE

Hyposalinity treatment:
This is where you reduce the amount of salt in the water to level that the fish can just about tolerate, but inverts can’t.  This is why you can’t do this in the DT.  Looking back at the lifecycle, you’ll see that the parasite will drop off the fish after a few days.  It heads to the bottom of the tank to reproduce, and during this stage it relies on osmotic pressure differences between the inside of the Tomont and the surrounding water to draw in nutrients and pass out waste products.  By lowering the salinity, there is no osmotic pressure difference (the SG within the Tomont and surrounding water is the same), and so it can’t feed, and dies.  Hooray!

I am going to refer out to another article to describe how to carry out hyposalinity correctly, as there is a lot of detail, and important points to follow:
Aquarium Fish: Applications for Hyposalinity Therapy: The Benefits of Salinity Manipulation for Marine Fish — Advanced Aquarist | Aquarist Magazine and Blog

One note I’d add is that it doesn’t use the scale we normally use in the UK for measuring salinity, so I’ll state it here.  You will need to reduce the salinity down to 1.008/1.009 for the treatment to work.  If it even momentarily creeps above this, you will likely fail…I know from experience ?
The treatment of the fish takes two weeks, and you need to leave another two to make sure it’s dead.  However, you can’t put them back in the DT yet!  We need to cure everything else, which takes 8 weeks.

Copper treatment:
Firstly, copper is highly toxic to inverts.  This is why you can’t do this in the DT.  I will not describe the process here, as it’s different for different chemical treatments you buy.  Simply follow the manufacturer’s instructions.
The treatment of the fish takes two weeks, and you need to leave another two to make sure it’s dead.  However, you can’t put them back in the DT yet!  We need to cure everything else, which takes 8 weeks.

How to treat everything else
This is the easy bit.  Simply leave the DT fallow (fishless) for 8 weeks.  That’s it.  Without a fish, Ich can’t complete the lifecycle, and so will die.
Important notes when carrying out these treatments.  Contamination from one tank to the other will mean starting all over again, so:

• Never touch anything in one tank and then touch the other without washing your hands/arms thoroughly first
• Never transfer any equipment between tanks i.e. nets, pipettes, algae clips, jugs, buckets for water changes, tongs, etc.

Prevention
I have learned my lesson, as have many others, and now have a quarantine tank (QT).  Everything I intend to add to my DT now passes through my QT.  That means fish, critters, corals, liverock, EVERYTHING.  It stays in there for 1 month, so I can observe any signs of disease, or potential unwanted hitchhikers, and deal with it appropriately.

Further to this, all fish and coral gets a dip, which will dislodge a great deal of unwanted parasites and hitchhikers.

If you have taken my advice, you will now have a second tank to treat you fish in.  Why not keep it and make it your QT moving forward?

Good luck!

Before I answer these, I want to say that I intend to write some more articles that will address a lot of these questions, which I will then link this one to.  Such as how to set up a hospital tank and how to set up a quarantine tank.  I will add 'dips' to the list 

1, Do you use tank water to set up the HT\QT and replace with new mixed water in the DT?

For HT, that's normally set up just for the treatment, then cleared down.  So, yes, I used DT water to fill mine, then top up the DT later.  I didn't have ready mixed salt water to hand, so moved a heater and thermometer to the DT and just switch off everything in the sump.  I made up salt water and filled it back up the next day and put it all back to normal.

For my QT, that was set up using brand new salt water, as I didn't want to contaminate it with Ich from the DT.  I bought some liverock from an establish system in my LFS so I had very little die off, and just cycled using some shrimp. This was also to make sure I did not transfer Ich from the DT, which would have defeated the purpose in having a QT.

2, for the canister filter do you start with brand new sponges or do you have them sitting in your sump until required?

I used an in tank sponge filter, but a canister would work too. 

When setting up the HT, you can use a sponge from your sump to jump start the cycle, and help deal with ammonia, but if you have to be aware that sponges in sumps are nitrate factories, so you need to be prepared to clean them out regularly if you want to keep one ready for HT use.

I didn't have one prepared, so instead, I used some magic potion .  I would not encourage this in any other situation, but in the case of a HT, it is useful to use something like AmLock, which prevents ammonia poisoning.  You need to be aware that this will mess up your test results, so do so, knowing that and you will be fine.

For a QT, I wouldn't use a sponge filter, just a couple of small pumps.  I'll provide more details in another article.

3, are water changes needed in thr HT?

Yes!  Lot and lot and lots of water changes.  You will probably have no bacteria to start with, so ammonia builds VERY QUICKLY.  It's also important as pH is unstable at low salinity, so doing daily changes keeps that in check too.

4, did you try more than one copper treatment brand?

I've not done it yet, as the hypo salinity method is less stressful for the fish, and after two attempts (I messed it up the first time), has proven to work successfully.

5, do you have to reduce the amount light? Or is that another myth.

Myth.  A little light will stop the fish being as scared if you walk past them though.

6, do you have the QT running all the time or set up when required?

All the time now.  I will use it as a frag tang once I have fully restocked.

7, This might be a topic for another thread, could you go into a little more detail on how you dip the fish\corals with pics?

Will do a video I think, as I have fish on order.

Thank you for your reply it is very informative and helpful I will be sorting out a QT ASAP unfortunately one fish has died and the other is not far from it. I have also noticed that this has only affected the regal tang and vampire tang yellow tang and cleaner wrasse but not the chromis fish or clown fish which seems really strange too

Possibly also a treatment of polyp lab medic may help lower the no's of parasite in the DT and is reef safe , may help along until QT / HT is ready .

If using the hyposalinity technique won't having the salt content so low kill off any good bacteria and animals in the live rock rendering it useless?

Ah thanks. I am using he hypo technique with no live rock just got a sponge filter (which I kept in my sump for a few weeks prior) and no live rock. I have also put in some Colombo bacteria balls in a little dispenser to help.

New to the hobby only been going since November what is an atu?

Salinity is at 13ppt at the minute. aiming for 14ppt so heading a bit higher too keep it in the middle of the 12-16 range.

There is also a powder blue tang hiding in the black pipe :)

There's really two approaches:

1. Remove all fish, treat them and leave the main tank fallow (no fish) for 12 weeks.
2. Leave them in, feed well, and hope they fight it off.  There's a theory that over time, the fish get better at fighting off a particular variety of the parasite, so eventually it can't complete the cycle and it dies out.  I'm not sure there's proof of this though.

You seem to be mixing the two.  The problem you may well face, is when you put the tangs back in, ich is still there, and they immediately pick it back up again.  Also, there's a good chance that within a week or so, all the other fish will be covered in it. 

My advice would be to put the other fish in the QT too, and leave the DT fallow for 12 weeks.