I hope this helps.
Everyone remember how I said the internet can be a good source of information but also a bad one when we listen to "so called experts" who only quote what they see rather than what they know. Which always leads to a new hobbyists being confused, so I'm going to try to help you here before we loose you to those "experts" :thumbsup:
It is my understanding that ich is always present in every tank, every ocean and on every fish but only hatches with a temp drops.
Myth: There are many factors that can cause an ich outbreak temperature alone has little to do with it.
First of all ich is not caused, it just is. An outbreak is the proliferation of a pre-existing parasite that has been given ample conditions to spread and when they spread they spread fast. Ich though normally present in SW lies dormant until conditions are met that allow it to grow. So in a way yes ich can seem to just come out of seemingly nowhere.
"Cryptocaryon is a parasite, and like most parasites it is very prevalent in the environment of the species it normally infects. Therefore, most fish are exposed to low levels of this parasite fairly frequently and are able to effectively fight off the infection without becoming seriously ill. In an existing tank in which the fish are healthy, the introduction of a new fish or a decrease in water quality or temperature fluctuations may stress the fish to the point that the Cryptocaryon protozoans that were present but not creating problems will then rapidly cause a more serious infection. "
In other words, Ich Cryptocaryon is present in all SW environments to the extent that the flu virus is present to some degree in the air we breath daily. If you are healthy you have little to worry about. With ich though there are some basic precautions.
Be careful where you get your fish from, if there is a high level of stress during shipping it is likely that less hardy species can contract ich. If the LFS says they would like to hold the fish for a while let them don't buy fish strait from the shipping box. I have seen this happen and I always warn customers to just wait. It has little to do with the ich and more to do with the fish that may contract the disease and spread it rapidly to plague proportions, if the fish is healthy to start with, it just won't contract ich or if it does it will get over it very fast.
So what have we learned? There is no magic temperature that causes ich outbreaks so that's a myth, it is the result of an imbalance in the systems and biotope. Temperature swings up and down repeatedly CAN trigger an outbreak so can salinity swings, and dirty tank conditions, no hiding places, no water flow anything that induces stress in the animals can trigger an outbreak. In the cases where we see someone return one of our fish after an alleged outbreak in there tank I can always track it to 3 causes 4 if it's a tang.
#1 The tank is not cycled enough (about 6 months before you can be 100% sure IMO)
#2 They bought fish that were in rough transit.
#3 They added a fish from another LFS that may have had a higher Cryptocaryon exposure rate.
#4 Not feeding properly.
So as for the sand contaminant that's why when we do sand swaps we need to be careful as some sand can theoretically have a much higher cysts count especially if it had recent outbreak. Again it's the health of your fish that matter not really the sand so don't participate in a sand swap unless you are 100% certain you are cycled out 6 months or more and you are not adding a highly stressed fish (example, tangs) to your system afterwards.