Priscilla one of the most common ways our fish get/contract many parasites are from other fish be they from a dealership or from a friends fish, as such it vitally important that all new fish are put through a period of QT.
How long must they stay there , we recomend a QT period of 8 weeks but others say 4 weeks but basically its down to you how long you QT for .
During this time if they remain un-treated then you have no garentee the fish is Parasite free so its best to at least salt bath in a short term salt bathor treat for parasites in some way or another
Otherwise Priscilla all you can say is I havent seen a problem "but it still doesnt mean they arent parasite free all it tells you is their arent enough parastes on that fish to become an issue" .
So you must have a QT system in which you can treat any new fish ( a stock tank of 100 gallons or more will do ith a UV-C and filter added) ,
"We have a purpose built fasility which is basically a 550 gallon imperial self contained pond", non of the water mix's with the other main pond.
Make sure the dealership you visit treats their fish prior to them going for sale any good dealer will confirm they have and tell you what they treated them with and for what.....
I mean they have too dont they, because after all we are their bread and butter, get a bad name and in no time people will desert them and they will go bust.
Believe me Priscilla word soon gets around who's' a good dealer and who is a bad one .
But the bottom line in all this is us the hobbyist it is you are making that purchase no-one else.
As such use your head look for problems in the vats, ask yourself are there any sick dead or dying fish in those vats.
If the answer is yes then simply walk away its not worth it.
Val and I are extremely vidulent when buying new koi and refuse even a split fin in a Vat,
Private sales between two indivual keepers are another way they get in we did a thread on how to go about these sales :-
https://www.gardenpondforum.com/threads/proceedures-when-buying-a-koi-from-a-private-owner.10505/
I remember getting into a heated discussion with dp about a koi given to one of our members who's pond was just next door but in the end that fish was QT'd in a school .
Now if you dont have a QT fasility/pond and your buying privately, be extremely careful who you buy your new fish from, making doubley sure that that person is very conscientious about treating all their fish to make sure they are parasite free before they sell them to you .
Do not take their word for it that they are parasite free, ask them exactly how they treated their fish for parasites.
If their answers are vague about how they treat their fish then alarm bells should start ringing .......
So if youve set your heart on it and really must buy it then assume the fish "WILL" have parasites then make up your mind and if you still want the fish QT it .
The reality is that, at least 80-90% of the time, the parasites are introduced to the pond from a new fish which was carrying with parasites.
Healthy fish can Live with a low level of parasites for many years and thrive
Why well this is because fish produce an enzyme in their slime coat that basically inhibits the reproduction of parasites.
If the the fish doesnt get sick or stressed then parasites are kept in obeyence and at a manageable level but If for some point the fish does get very stressed or sick it will stop producing the said enzyme and the parasites seeing their chance then go for it reproducing unchecked and become a major issue in your pond
So try to reduce stress in your pond remember to QT first, then hopefully you cannot go far wrong ,
Dave