[scvkoi]

Scoliosis -- Lateral (sideways) curvature of the spine is what we are talking about here.
Here are some of the known causes:

Electrical – Yes as stated above the cause could be electrical, but not the kind that would cause a Ground Fault Interrupt (GFI) breaker to trip. What I’m talking about is stray voltage, which is another name for electrical ground current. Electrical ground current is electrical current returning to the substation through the earth. Electrical ground currents occur when the utility's wire is no longer the path of least resistance back to the substation. Electricity always follows the path of least resistance. Once it is off the wire, that path often includes plumbing, bodies of water, and animals, in addition to the earth.

To determine if stray voltage is involved, you can test as suggested by B. Scott or with an AC digital voltage meter (available in the U.S. at places we call Radio Shack) set at the mV setting. You place the negative (black) probe to the ground of your electrical outlet and the positive (red) probe to the actual pond water (a metal rod may be used to extend the probe if needed to reach the body of water). If there is no stray voltage the reading will be zero to .1, readings of .1 to .5 and you might have stray voltage, readings above .5 and you definitely have stray voltage (note: the reading on the digital voltage meter will probably fluctuate). The amount would be so small that one would hardly notice it, but since the koi live in the pond exposed to this very small amount of voltage 24hours a day 7 days a week, etc. Then over a period of time you will see the impact, in bent backs.
What in the Koi Pond system could be causing stray voltage? Here are several possibilities: The neutral conductor is grounded at more than one point in the system, old style pump motors that use the motor frame as a conductor, old 220 volt pumps that connect the neutral to the pump chassis, high resistance shorts to ground in electrical panels and boxes, incorrectly wired motor starters and defective equipment.
How do I ensure that I am not creating stray voltage? Select electrical equipment that will not generate excessive ground currents. Electrical motors on pumps used by koi hobbyists are primary sources of leakage current even when they are UL approved and working correctly. All electrical equipment needs to be tested for leakage before it is installed. If the leakage current is in excess of 1 milliamps (0.001 Amps), then the equipment should not be used. Have a competent electrician check to ensure that the neutral to ground is only at one point throughout the system.

So then what about the GFI, and why wouldn’t it be tripped by this electrical current? Well the GFI measures the current going out of the black wire (hot) and coming back through the white wire (common). If some of that current going out through the black wire doesn't come back through the white wire the circuit inside the GFI box senses that you are about to get fried (via the unbalance of the outgoing and returning current) and shuts the circuit off within about 10 milliseconds!

In short, stray voltage is voltage between the neutral and ground, while the GFI deals with voltage between the hot and neutral. Thus a GFI would not solve a problem with stray voltage since it deals with a different voltage.
So am I a competent electrician? Heck I am not even an electrician, but when I was with the local saltwater club a few years ago, a number of members reported a slight tingling sensation when they sometime touched the water when performing water changes even though they had not spilled any water and had GFIs installed. Fortunately in the club there were several competent electricians in the club who knew what was going on. What was happening was that many saltwater hobbyists were installing many pumps (power heads, pumps for filtration systems, pumps for foam fractionators, etc.) that were not properly insulated and/or grounded and thus were generating stray voltage. The solution was either unplugging various pumps until the stray voltage disappeared or installing stainless steel ground probes. Well more than a few people wound up buying stainless steel ground probes to resolve this problem. Other saltwater hobbyists confirmed this problem and its resolution across the country. No offense to the electricians of the world intended, but I only mention competent electricians, because some will give you funny looks if you ask about stray voltage.

Since mscwd’s description points to severe bend backs and previous history this is the most likely culprit.
Note: I live in the U.S. and the coloring on the electrical wiring I refer to is what we use, please check with your local electrician for your country’s electrical color coding.

Vitamin C Deficiency – This causes a slight curvature or kink of the spine. If the koi food you are feeding only has ascorbic acid (Vitamin C) and not stabilized ascorbic acid (Vitamin C) and has been sitting around awhile then the koi are not getting the proper amount of Vitamin C.
Eight years ago I had a Hi Utsuri that developed scoliosis that may have been a result of the koi food I was using at the time.
Well we had not had electrical storms recently that would lead to this as the cause. So I tested for stray voltage and found that not to be a possible cause, nor had I recently treated with any medications so ruled out these causes.
Research into scoliosis indicated that a deficiency of Vitamin C as a possible culprit, so I wondered if a deficiency of Vitamin C brought the koi to this state then a larger amount of Vitamin C, might reverse the situation. So I spent the next year and half feeding my koi home made food that consisted mainly of spinach, broccoli, and dark lettuce with added stabilized ascorbic acid (Vitamin C) from the health food store. This was fed to the koi in the morning and evening during the heavy feeding season. Slowly but surely this did result in the koi spine straightening out. The koi did not have any adverse effect afterwards. The koi was sold at auction late last year I think, but I don’t know who bought it. I have not had a scoliosis problem with any other koi before or since then, so I am not sure if the same treatment would provide the same results. Unfortunately this was before digital camera being affordable, so I don’t have pictures to post.

Other possibilities include adverse reaction to drug treatments or improper injection of the koi, but since this is occurring over a period of time and to a number of koi these possibilities are not likely.

Hope this helps.