Urgent! Sick koi!

[Mayank]

Update. Got the fish out and did water changes x 3 and used ammo lock until the ammonia level came down to barely detectable. Unfortunately my big koi did die. Thankfully the other fish, all much smaller seem to be doing well. I dosed with some bacterial additive this afternoon. So now the pond has got 2 pumps, 1 huge biofilter, a smaller biofilter and 2 aerators going. It's been in the 90's for the last week and no changes in the forecast. I thought the pond (between 1500-2000 gallons) was adequate for the number and size of fish I have. I've seen smaller ponds with much larger koi in larger numbers that seemed to be doing great. Did I see somewhere that recommended 1 koi per 1000 gallons? Anyway, I'm adding more floating plants as well and will certainly be monitoring and testing water quality much more frequently!! Again, thanks for the prompt responses and great advice!!

So sad to hear about your big koi dying.
Just curious. When you said NH4 was way high. How high was it? What is it now? How are you testing?

 

[Lisak1]

There's a common axiom that you need "1000 gallons for the first koi". I think the idea there is to help people understand that koi don't belong in ponds that measure in the hundreds of gallons. And I wish I had a dollar for every time someone has said "I have X number of SMALL KOI". There's no such thing as a "small koi", just young koi ready to explode in size! Like @Jhn said - if you see a pond that's loaded with big fish, look around for the filtration shed - it's there somewhere! If you put enough filtration on a pond, you can overstock. But then your pond is more of a chemistry experiment as you have to keep careful track of the water parameters and make adjustments as the situation warrants.

Here's an interesting chart that I like to share with people - check out the difference between the waste produced by a ten inch fish and a 30 inch fish. Would you rather have one 30 inch koi or 20 five inch goldfish? Not all koi will grow to 30 inches of course, but most will get to 18 or 20 inches very quickly! So even the old "one inch of fish per so many gallons" rule isn't applicable to a pond.