Wow - let's get the forum off with a bang I do not mean to insult anyone's intellligence, but this forum is for all levels of the hobby - beginner to advanced. So my reply will be basic for some, but hopefully a refresher for all!
Too often than not, a fish, invert, live rock, etc is not quarantined properly and is added directly to the display tank - introducing all kinds of opportunistic pathogens.
Certain fish are definitely more susceptible to marine ich (crytocaryon irritans) such as the tangs and puffers
A common thread you will hear, "my fish was doing fine for a couple of weeks & all of a sudden white spots appeared". The spots can often appear after there has been a change in the tank (a stressor) such as sudden water parameter changes (change in pH, alkalinity, ammonia, ORP, temperature), new tank mate added, mating/courting, aggressive substrate vacuuming or tank redecoration. Chronic stress (stress that goes on for a prolonged period of time - days - wweeks - months but doesn't acutely cause behavioral changes in the fish). Examples of this would be chronic suboptimal water quality (chronic low oxygen levels, ORP or pH), inappropriate or tainted/spoiled food, unwitnessed aggressive tendencies or inappropriate tank mates. These chronic conditions put strain on the fish's immune system where once it was able to fight off the pathogen/parasite, now becomes infected.
In order to understand the parasite, one must understand the life cycle of the parasite. When one sees the actual white spots, that is the adult often referred to as trophonts. These will mature and encapsulate themselves and fall off of the fish to the floor/substrate. The encapsulated cysts are now called tomonts. Tomonts will divide/multiply into small ciliated (hairlike) organisms. They are now called tomites. Tomites swarm the tank looking for new hosts (fish). This parasite bores into the mucosa of the skin, fins, and gills and then continues its life as a trophont.....and the circle of life continues. This process takes several days so often hobbyists will see spots "come and go". The ich is not gone but merely entering a new cycle and multiplying.
Symptoms besides the white spots include:
1. scratching and eradic/frantic swimming(fish trying to rid itself of the boring pest).
2. increased or heavy breathing, with the fish tending to stay at the water surace, close to water return pipes/powerheads or in airstone bubbles (parasite in gills affecting respirations).
3. increased mucous production (slime).
4. eye cloudiness (especially with puffers).
When left untreated, this parasite infestation can lead to other secondary opportunistic infections - bacterial infections like fin rot, red patches, ulcerations.
The display tank is infected with the parasite. If the fish has a strong immune system, the fish has a chance (I am not a betting person so don't ask me to place odds or chances) of overcoming the infestation. With that being said, the constant presence of the parasite and reinfection until (if ever) the fish develops an immunity/resistance can cause chronic stress and the fish can become victim to something else (bacterial, fungal or worm infestation.) Adding new fish at this point, even if they have now been properly quarantined , to this system is risky. They too will be exposed and they will surely become infested.
To rid the tank of the parasite:
I would recommend removing ALL of the fish and place in a separate bare bottom quarantine tank. By removing the fish, the parasite does not have a host to complete its life cycle. The tank will have to remain fallow (fishless) for a minimum of 4 weeks, I prefer 8 weeks. Another option would be to treat the main tank with copper. I do not recommend this especially if this is a reef, one with liverock or substrate. Copper will KILL inverts, corals and the micro fauna on the live rock. Substrate will leach/absorb the copper and the therapeutic treatment levels needed to cure/rid the parasite will not be achieved. There are many advertised/marketed "reef safe" ich cures - I don't recommend them. If you visist enough sites, talk to enough hobbyists, you will find those that highly recommend these products and then others that have had miserable or disastrous results....this is true about every aspect of life and science. I do not gamble when it comes to the life of an animal, fish or human.
What to do for the fish:
All the fish need to be treated in a separate quarantine/hospital tank.
*Hyposalinity (S.G. 1.009 - measured with a refractometer at least daily) for a minimum of 4 weeks. The parasite can not live in hyposalinity.
* The tank needs to be bare bottom (no substrate) with hiding spots for the fish. Hiding spots should be things that can be sterilized after each use - pvc pipe/couplings or those plastic caves/decorations used in freshwater tanks.
* There should be some source of biological filtration. I prefer air driven sponge filters that can be cycled or maintained in your display system's sump or a sponge or bioballs in a HOB (hang on the back filter). Both of these can be quickly removed from the display system and added to the QT.
* Additional aeration (air stone or power head pointed from the bottom of the tank to surface) will also be needed as these fish are stressed and this causes an increase demand for oxygen.
*Another option is to treat with copper in the quarantine/hospital tank. Copper has been proven as an affective treatment. Copper too is an irritant and fish breeders have found it to affect fertility. Copper kills inverts, algaes, corals. Some fish (scale-less fish, elasmobranchs) are very sensitive to copper and do not do well. If you use copper, in addition to the other water parameters, you must monitor the copper levels daily to prevent toxicity as well as subtherapeutic levels (low) as both extremes will affect the outcome/cure.
Whether you choose hyposalinity or copper treatment, you must monitor the water parameters in the tank at least daily as pH, ammonia and nitrite levels can change rapidly. This causes additional stress which can affect the success of cure. Frequent water changes and the addition of buffer to maintain pH are required.
Improving the fish's immune system will also help. You can do this by the addition of beta glucan, vitamins and garlic to the fish's diet daily while in treatment.
Watch closely for secondary infections as mentioned above. Antibiotics may need to be added. I would not recommend treating with antibiotics prophylactically or "just in case" as this can add stress or lead to a drug resistant pathogens.
Don't like these proven methods of cure- The only other option is to break down the tank & start over.
Wow - I now have writer's cramp and this is just the "reader's digest version"
Everyone still awake?
Best possible advice - Quarantine everything, you won't regret it!