Question
I have a small backyard pond with 6 koi in it.  The fish are young and only measure 5-6 inches. The pond is 6x6x3 and is in sun for half of the day.  It has an upper pond with a waterfall.  It also has a bio-filter.  I do water quality checks every 2-3 days because it is a newer pond. My question is regarding one of the fish.  All of the fish are very energetic and all are eating well.  I have noticed within the last week however that one of the fish is developing spots.  It is a white fish with a red head.  About a week ago, I noticed 2 black spots on the top of its head.  I was just out feeding, and noticed  3 more random spots on the body.  Can koi get spots as they get older?  Do they change color or is there some kind of disease this fish may have?  Should I quarantine it?   I would appreciate any help you can give.  Thank you.

Answer
Dear Chandra,

Thank you for your question.
Congratulations are in place as a pond owner.
I wish you al the best with the koi.

The issue is that, if you see black spots, however really in the scales it can be "Sumi", or black scales. this is common with koi as they grow.
That's also one of the items why koi are very expensive if they are clean pure with with no black spots.

However if the behavior is different it can be parasites or wounds and bacteria.
To be sure you need to catch the koi and look very closely.
Do you see parasites or wounds?
Best way, if this only is visual with one koi, is to place the koi in quarantine and take scrapes for microscope tests. If this is not possible you can buy anti parasite products.
But be carefully with medicine and the best way is to only use them if you are sure of the disease.

for extra knowledge to check for parasites: http://www.koicarp.org.uk/koi_parasites.htm

Good luck and please keep me posted.

Kind regards,
Raf