Paul B
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This seems simple to me. You don't see ich on most fish in a store because of the copper or whatever else they use to suppress it. But the fish are not there long enough to completely eradicate it. When you put the fish in your copperless system, the parasite shows up.
Being your tank has been ich free in 5 years, any parasites in there would have died out by now. But the fish also most likely lost their immunity to ich because they have not been exposed to it and immunity doesn't last forever. No one knows how long it lasts but many guess and I am sure it varies depending on the species temperature and a host of other things.
The key is to introduce ich occasionally or at least every year or so. You can add a fish if you like.
Now your fish may all come down with it or they may still be immune. No one can tell you.
I have not seen ich or any pathogen in my tank in decades but I do add fish occasionally. Many of the fish we keep have a lifespan of 8 or 10 years and when they die, I put in something else. I have about 30 or 40 fish so every few months something will reach the end of it's life. I have a couple over 30 years old so they may go in a few years.
I never quarantine or medicate anything because I want to be sure I am adding parasites to keep up their immunity. Another key factor is to feed living gut bacteria once in a while like from live worms or shellfish.
Good Luck
Being your tank has been ich free in 5 years, any parasites in there would have died out by now. But the fish also most likely lost their immunity to ich because they have not been exposed to it and immunity doesn't last forever. No one knows how long it lasts but many guess and I am sure it varies depending on the species temperature and a host of other things.
The key is to introduce ich occasionally or at least every year or so. You can add a fish if you like.
Now your fish may all come down with it or they may still be immune. No one can tell you.
I have not seen ich or any pathogen in my tank in decades but I do add fish occasionally. Many of the fish we keep have a lifespan of 8 or 10 years and when they die, I put in something else. I have about 30 or 40 fish so every few months something will reach the end of it's life. I have a couple over 30 years old so they may go in a few years.
I never quarantine or medicate anything because I want to be sure I am adding parasites to keep up their immunity. Another key factor is to feed living gut bacteria once in a while like from live worms or shellfish.
Good Luck
I also have no way to test it but for the first 45 years, I didn't live on the beach. 
