Help! Koi not doing well

[MMontgomery]

My husband and I inherited a pond with our new house. It is roughly around 500 gallons we are guessing. He did a water change yesterday and also used an algae treatment. Later that day we noticed the fish not doing well, acting very lethargic and breathing heavily.
Looking more into it, it says that it can deplete the oxygen levels in the water. We ran to get a big air pump and air stones to get more water movement and hopefully more oxygen in the water. He did a change of water last night trying to get some of the algae fix water out. We lost some fish overnight, mainly goldfish.
We have two larger Koi that were still holding on this morning but don't look great. He tried another small water change and has been using the dechlorinator.
I am guessing the mix of algae treatment, removing good bacteria etc. has shocked them.
What can we do to salvage these guys?
I feel absolutely terrible and am so heartbroken.
We tested the water and yesterday was PH 6, and everything else read at 0 or low (GH, KH, NO3, NO2).
I am at a loss of what to do and now one of the larger Koi is not looking good.

 

[Mmathis]

Hello and welcome!

To start out, 500 gallons is way to small for koi. That could be the root of your problem. The problem being poor water quality. Too many nutrients in the water and your biological filtration can’t keep up — that’s where the algae is coming from. Algae isn’t a bad thing, as long as you have good filtration and adequate water movement. We don’t recommend using chemicals such as algicides, but rather, taking care of the root problem, which in this case is overcrowding.

Exactly how many koi do you have? You might want to start by rehoming them — ideally, 1000 gallons is the minimum size for a single koi. I know that’s not what you want to hear, but until you reduce the number of fish, you are going to continue to have problems.

 

[MMontgomery]

The pond has been established for many many years, the previous owner said that the only reason they lost fish before was due to a racoon. We have plans to make it at least twice the size within the next year.
I think the major water change with the algae treatment caused the low oxygen? The algea honestly wasn't bad. My husband was just doing what he thought was best and the previous owner left behind a bunch of supplies. Will take this as a learning experience but totally devastated it had to happen like this.
We got some stress coat and pond salt. There is only one Koi remaining unfortunately but seems to be doing better.
There were two medium Koi and some goldfish.
Thank you for your response.

 

[brokensword]

The pond has been established for many many years, the previous owner said that the only reason they lost fish before was due to a racoon. We have plans to make it at least twice the size within the next year.
I think the major water change with the algae treatment caused the low oxygen? The algea honestly wasn't bad. My husband was just doing what he thought was best and the previous owner left behind a bunch of supplies. Will take this as a learning experience but totally devastated it had to happen like this.
We got some stress coat and pond salt. There is only one Koi remaining unfortunately but seems to be doing better.
There were two medium Koi and some goldfish.
Thank you for your response.

it would be wise to listen to what Mmathis stated; by putting in (more) chems, you're only going to have (more) problems. A larger pond will buy you time but if you don't stock correctly and have sufficient filtration, the problems will just reappear. 1000 gallons for ONE koi, 250-500 for each additional. So, plan your larger pond with those number in mind, stop putting in chemicals (except for countering any chlorine/chloramines in your source water), use lots of plants (protected from the koi, if you have to), consider bog filtration, and then you can sit back, relax, come on the forum and help others like yourself get past methods that generally hurt the hobby.

 

[j.w]

@MMontgomery

 

[mrsclem]

Let me add to what the others have told you and say do not add salt! It will do nothing for the pond. Its good as a treatment in a quarantine tank but not as a pond additive. .

 

[WaterGardener]

Your pH is too low. It's acidic. The beneficial bacteria you need for the nitrogen cycle can't live in acidic water, so you need to raise the pH. I suspect you have had a pH crash and that is a deadly thing to fish. Your fish have outgrown the ability of your system to buffer the acid being produced in the cycle so you need to add more buffer, like baking soda.

PH is easily raised with baking soda, but it should be done slowly so as not to shock the fish even more. Add a small amount of baking soda at a time, 1.5 ounces per day should help. Mix it with pond water before pouring it in.

You need a KH liquid testing kit to measure the amount of alkalinity, the buffering for the acid. There should no less than 100 ppm of KH and more is not a problem. I try to keep mine around 200 or more. KH will keep the pH stable and prevent another crash. But it will need to be checked and boosted occasionally since it will be used up over time.

The 1.5 ounces of baking soda should raise the KH almost 1 drop per day. When you get to a sufficient amount of KH, the pH should be around 8.2.

I agree that your pond is very overstocked. That has contributed to the pH crash.

 

[poconojoe]

Agree with everything said.

There are so many threads here that are titled "all my fish died" [after adding algaecide or some other miracle muck reducing snake oil].

No salt either. These are fresh water fish.

Ponds are not swimming pools. They should not be sanitized. They need all those microorganisms in the water and filter to function properly.
They are not aquariums either.

We strive for an all natural form of ponding.

Plants are a neccessary component of the pond's ecology. They help filter out excess nutrients in the water. Add lots and lots of plants...you need them.

A lot of people wonder why all of a sudden they have problems they never had before.
What usually happens is the fish keep growing, so you hit a tipping point. Too much of a fish load for your size pond and filter to handle.

Add lots of plants to combat the excess nutrients.