Question
I filled my new 250 gallon pond with fresh water from my well, in rural Virginia. waited two months, then placed 13 koi carp. They are mostly small, less than 4 inches, one is 6 inches and one is 12 inches but friendly not aggressive. I placed a net over the pond to protect them from wild life. If I install aquatic plants, critters will be able to gain access, is it OK for the fish if there are no plants? The 6 inch white koi has "ick" which I have treated with appropriate medication. Why does "ick" and the black spots afflict these fish, is it the well water. I have not tested the water as I assumed well water was perfect(?) Maybe I should, but what am I looking for? Many thanks for your interest and any help!

Answer
First off, 13 fish are way too many for 250 gallons.  There is just about no way you could maintain a healthy water quality in this situation.  If you want plants, you can get some that will stick out between the holes in the netting.  Plants do help to add oxygen and reduce nitrates.  Ich is a bacteria that is brought on by large temperature or pH swings.  Some strains of Ich are resistant to the medications.  I'm not sure what black spots you are talking about.  I'm not sure what about well water is perfect...in my opinion, all tap water is equally bad (but workable) and every tap water is wildly different from the last.  I've never seen two wells water was exactly the same.  This means it could be either better or worse than tap water.  There is no reason not to test the water.  Monitor pH, alkalinity, NH3/4, NO3, and NO2 (NO2 unnecessary after a couple months).  There are several great articles on pond water chemistry online and many of my previous answers have information detailing this topic.