Hello,
Let me start off by saying I'm new here, and new to koi care. I purchased a home that had an established koi 1200gal pond with 13 koi ranging from 5"-16". I've been taking care of the pond since November of last year.
It's been quite the learning curve. We've had a few mishaps and learning lessons along the way. About 2 months ago I was out of town and my girlfriend left the hose on overnight while topping off. Unfortunately, one of our largest koi died the next day.
Two weeks later, I left the valve slightly open in the filter after back fishing, drained 60% of the pond overnight. I refilled the pond and added dechlorinator.
All was well for another two weeks, but I did notice my favorite koi was "acting different" maybe a little more sluggish, but nothing major just different.
The next week, I completely cleaned the filter, because I was.getting more particulate than usual and it didn't get cleaned after spawning this summer.
Two days later the koi that was acting strange is now sitting at the bottom, isolated, and not eating. I watched him closley the next couple days. He started clamping so I decided to take action. I tested the water:
Amonia -.5
Nitrite - 0
Nitrate - 40
pH - 6.8
I thought that's probably not too unusual after cleaning the filter. So I put some beneficial bacteria in to help with the amonia break down. I also salt treated the pond, which I have never done before. My next tests showed drastic improvemts in water quality:
Amonia - 0
Nitrite - 0
Nitrate - 20
PH - 6.8
It's now been 5 days since he stopped eating. He has slowly showed improvments, but still has not ate. He no longer seems to be clamping he is swimming around more, but still isolated and now has moved up towards the top of the pond. He is not gasping for air at the at the surface just sitting near the top. His mouth and gills are moving constantly. He is starting to lose color, I'm assuming from malnourishment.
I don't really know what else to do at this point. I've considered three possibilities.
#1) a parasite of some kind. I don't have the capability of doing scale tests. Thinking of treating the pond with broad spectrum. Thoughts?
#2) He has permanent Gill damage from the chlorine that was put into the system by the over filling that night. However, no visible Gill burns. Considering installing an airration system, which I want to do any way, but might make that a priority now.
#3) Is he stressed from the environment changes and is treating the pond making it worse?
SIDE NOTE:
The previous owners put copeous amounts of chemicals in every week. I've weened the pond off all the chemicals slowly. My water quality has had many fluctuations in that process, but has never been better than now. I've got a nice furry algae coat, and overall the system seems to be doing very well.