Question
QUESTION: Hello.  We built a 14x16 pond about 1 month ago.  Before getting any fish, my children put in many minnows and tadpoles from a creek that runs thru our yard (ohio).  We bought 10 small koi, all dies but 2 over first 2 weeks.  We then bought from a local person 10 large koi fish.. 3 at about 2 feet long or larger. others 8-12 inches.  She had had them for years but in a tiny 3x5 pond.  They seemed to do great but we noticed all fish rubbing bottom of liner in a wrinkled spot.  Then today one large fish was dead on bottom.. We noticed other large fish had whitish spots and lots of muccuse (not sure spelled that right) all over them, sore spots I would call them, fins with sore spots, and another large one died.  We are giving them salt baths to try and help.  Did the minnows or tadpoles create all these problems.  We are also emptying out the entire pond and starting over.  But other 2 larger fish have spots/sores and seems to be a little better than two that died.  Any thoughts!!!! thanks.

ANSWER: I'm sorry about your fish.  While the minnows or tadpoles could have brought in various bacteria or funguses it sounds like something more dire is afoot.  I suggest not adding more fish until the pond is stable (no deaths) for at least a month).  When you add fish, the smaller koi usually are less likely to die from problems.

Have you tested the pond water?  You should test for chlorine, pH, hardness, alkalinity, ammonia, nitrite, nitrate, and oxygen but pH is perhaps the most important.  The "mucus" that you're seeing is probably the sloughing off of the fishes' slime coats.  There skin is literally rotting off.  That normally happens when something is wrong with the water such as acidic pH or poisoning (which could be almost anything from chlorine and ammonia to pesticides and medications).  The sore spots may be rotting flesh due to water quality issues, or they could be bacterial or perhaps fungal in nature.  If they're fuzzy, it's fungal, otherwise probably bacterial.

How are the minnows and tadpoles doing?  I would expect them to have problems if the koi are as well.

Aside from testing your water, I suggest doing a 10% water change once a week.  Be sure to add dechlorinator and some pond salt at 0.05% which deters a lot of bacteria and funguses.

You may want to contact your local koi club.  They can often come out and access the situation.  They can test the water and do scrapings of the fish to check for parasites.  It doesn't sound like a parasite problem to me but that can't be ruled out.

Also, check out http://www.koivet.com
They have a forum and can better help you with koi health issues than I.

Good luck!

---------- FOLLOW-UP ----------

QUESTION: We have a water testing kit and it all came back showing a little high on hardness, we have well water, alot of rust and limestone.  We added water 3 days ago and today is when we found the fish dead, but noticed all fish we have put in scratching on the bottom of liner.  The minnows/tadpoles seem fine.  Smaller 8" fish look good, as of right now.  Just the 2 larger ones had white spots, fins rotten partially, sores that look like an open sore and when I took them out of pond and put in a tub of water, you could see a film on them coming off.. like slim I would say.  Seem to all happen pretty quickly, but since we lost 8 small koi's within first couple weeks of pond, I read minnows can carry diseases and thought maybe that?  We do have rust in our water?  It is well water.  I have 3 lily's and some tall grasses in the pond?  We also noticed alot of algae coming on quicly

Answer
What was the actual hardness reading?  I have well water but mine is super soft.  If the water is extra hard with a high pH, it could be harming the fish.  The water may need to go through a filtration system before being safe for fish.  If the well is putting out rust than both your family and the pond animals may be getting too much iron.  Also, if you have copper pipes like we do, the copper levels might be high.  High copper can cause lesions on fish.  You should have the copper and iron levels tested.

Scratching or "flashing" sometimes indicates parasites but someone recently told me that a pH that is off is the number one reason for flashing.  

It definitely sounds like the big koi were sloughing their slime coats which indicates something is way off.  Have you kept fish before in aquariums or ponds?  Were there problems?  

Minnows certainly, like any fish, could have introduced a parasite, bacteria, etc.  Those problems though don't usually result in as much bodily damage as you're describing.

Algae in a pond is normal and actually good in many ways.  See http://www.fishpondinfo.com/plants/algae2.htm for more on controlling algae, etc.

I think you need to find out the exact pH, hardness, alkalinity, copper, and iron levels in your water.  I'm betting something is way off.  If they are ok, then I would lean towards a bacterial infection of some sort.

I again suggest trying to contact your local koi club or at least asking at the koivet forums for more advise.

I was talking with a koi woman a few weeks ago, and she thought copper might be a problem in my pond.  Since then, I've been adding stuff to detoxify heavy metals.  Here's the link she sent me about copper:
http://mywebpages.comcast.net/jnorth71/koi/

I wish I could help more.