I have a biofilter on the pond

Gotcha. But yes, I would say you had far too many fish for a small pond. Or rather too many koi. The same number of goldfish would be fine.

Sorry to hear about hte loss of your fish- its truly awful to lose even one let alone so many. What do your water quality tests say? Not that you have to be testing all the time but its essential for times like this - as one of the steps in figuring out what's going on.

I do need to agree with above posters. I like this guy's advice about fish and ponds - as he points out the old advice about gallons per inch of fish is totally not applicable to ponds/large fish. A koi that doubles in length from 13 to 26 inches increases weight from 1 lb. to 13 lbs- in other words produces a LOT more waste - more like 13x as much, not double. https://theponddigger.com/blog/?p=727

I have been using the testing strips the nitrite and nitrate levels are low I have hard water so the Ph is a little high but not enough to hurt them from what I've read.

My fish were not full grown I got them as 3" babies and the largest was about 10"

Emily,

The above posters are right about your pond being too small. You can reject their advise and kill more koi, or listen to them and buy goldfish instead.

In any case, sorry about the loss of your fish.

Emily and all, the guidelines and recommendations about pond size, filtration, etc are there because following them greatly increases the likelihood of success and pond health... its meant with kindness - I for one am certainly not trying to be pond police!

In yours and many similar situations (I was going through this myself a while back) there are so many variables its next to impossible to say with any certainty what happened - indeed there probably were many factors at work - ie poor water quality can lead to weakened fish immune systems and incidents of parasites and disease (which spread more rampantly in crowded conditions).

Btw ammonia and nitrite should be zero, not "low" - nitrate can be OK but not if excessive - and yet another variable I would mention here is - were the test strips accurate? They can be notoriously inaccurate and really only serve to alert you that something might be off - the chemical tests are more accurate. Other possiblies -toxicity? Neighbors spraying pesticide, sudden ph changes due to heavy rain? Chlorine filter on your hose suddenly not working - all those could cause gasping. Dissolved organic compounds is another manifestation of poor water quality that can weaken fish health.

Also one other thing you should know is that many dealers and manufacturers chronically overstate the capabilities of the equipment and what it can handle.... so when you say adequate oxygenation or good filtration - well, according to who? Without us getting specific about gallons per hour, what filtration, water changes, etc. its hard to say if we agree if its "adequate" or not.

I don't know how many years I was limping along with a stupid box filter that was said by both the manufacturer and the dealer to be good for a 1000 gallon pond - in reality maybe 100 at best. Technically , sure its possible to maintain fish health in crowded conditions but you really do need to know what you're doing and understand the concepts, nitrification cycle, and have expertly designed appropriate equipment (which tends to be expensive or diy). Or if you have your heart set on a large koi population, you ll want to enlarge and increase the capacity of your pond and equipment.

In the meantime, the best advice I think still is stock your existing pond very conservatively - get 2 or 3 goldfish at the very most for friends for your remaining koi- that would be appropriate for 850 gals.....assuming good flitration/water turnover of course, I quite understand the attraction of koi and it would be cool to have your remaining big guy but with only a few goldies as buddies so he has room to grow and excercise. Seriously! May look sparse now but trust me - with good water conditions and good quality food the big guy will grow to 2 feet or more in no time at all and the goldies to 10 inches (btw the comets and shubunkins can look very koi like with beautiful markings and long flowing fins).

Before adding new fish I would do several large (but gradual) water changes over a period of a few weeks though to make sure whatever was in there was gone, and also reassess your pump and filtration capacity.... either online or find a pond guru who knows his stuff. I had to look out of my city for that. And it goes without saying, quarantine new fish. Best, Cathy in WI.

Just a thought... If you lost them all at one time, I'd venture to say there was an event that caused it that quickly. Did you add anything new to the pond just before this happened? Put up a new cover over it? Add a decorative do-dad? Sometimes a seemingly innocuous thing is what causes it. Any metal, new treated fabric or treated wood?

I think the suggestion of a toxic substance being introduced into the pond is a good one, but I would also suggest it's far more likely her pond just reached a "tipping point" where things were stable until just one factor changed. In this case it may have been extreme heat. It may have been a thunderstorm, which can cause oxygen levels to crash in a pond.

It's also been an observation of mine that smaller ponds are much more difficult to manage and maintain than larger ponds. They seem to be always leaning towards becoming unstable. The lower volume of water means smaller variables have a larger impact I would guess, but in any case this pond is definitely too small for 9 growing koi. Some people would suggest it's even too small for one koi.