last Thursday I introduced two new fish to our pond (250 gallon ) one golden Koi and one golden orph. the total stock in the pond was then 14 fish three of them very large. after returning from a weekend away I found one of the Koi dead and removed it from the pond. The next morning I had another fish floating, whilst we were looking at the fish the wife watched one of the fish stop swimming and just floated to the surface right in front of her eyes. On doing a water test I found the toxic levels reading mid chart high so decided to change the water as it had also clouded over, I also cleaned out the filter after returning all the fish to the pond last night they seemed very docile and gasping for air. I put this down to the change of water temp and left them for the night, this morning I have found all but two of the fish dead and on close inspection of the fish I can see a very small red worm in the water around the bucket they are now in, yet their appears to be no discoloring or blemishes to the fish . what can I do to rectify this problem

Hi and welcome to the forum.
I notice you are in the uk.
14 fish is overcrowding your pond. One koi needs 250 gallons just for itself.
I think what happened is either the new fish introduced a parasite or bacteria to your pond or the new fish tipped the balance and your filter couldnt cope so the ammonia may have started to climb.
Do you test your pond at all for ammonia, nitrites, nitrates and ph?
Is there additional air going into the pond via a waterfall or aerators?
With the really warm weather a pond can become oxygen starved really quickly.
I dont know what the worms could have been but there is a type of internal worm called callamarus.

How often and how much were you doing water changes and how often were you cleaning your filter and how?

Hi Sandy
thanks for both the welcome and the swift reply
I do test the water with a test kit and all the ammonia, nitrites, nitrates and ph levels since the water change are perfect
I have only changed the water because I tested the water when the koi dies yesterday morning and found the toxic level to be .05mg/1 and the advice was take action to bring it to zero
I have a large filter box which I cleaned out by stripping it down and jet washing the box and scrubber pads and washed out the gravel trays in warm water. The filter allows the water to re enter the pond from two ports and falls about 6" to the surface, we also have a water fountain in the centre of the pond.

I failed to mention that the Koi are only small.
the worms are very tiny, red, about 2mm in length.
I think you may be right in saying they have been introduced by a new fish .
regards als

ok folks
all the fish are now dead so whatever caused it won the day. I have found out that the little red worms were not the cause so I am now at a loss.
I don't know whether to shut the pond down or start all over again
I guess you can move this thread out of the 911 area. thanks anyway

So sorry you lost all your beloved fish .If it was me I would empty pond scrub it all clean and start again.
Don't let this tragedy discourage you.Just monitor your water parameters weekly once its all set to go again

Finally a big

You may have disrupted the filter by cleaning it with water straight from a tap as this can kill the bacteria, which may be the reason for the lift in ammonia.
Although this wasnt the main cause for the deaths.
Usually if fish die in a short amount of time with no known cause, oxygen deprivation is usually the cause.

Al,

Sorry to hear about the losses. You've learned a lot here, which I understand isn't good because there were a lot of deaths.

First question did you QT those two fish before introducing them into your pond? When introducing fish into an existing invironment (pond or aquarium) you should QT them for at least a month. This way you can watch them and make sure they are not going to introduce anything to the other fish and either make all sick or make all die.

You were way over stocked for that size pond. Koi need 250 gallons per fish. The reason for this is they are dirty fish and produce a lot of waste. Of course they also grow to be up to 3 feet with good water conditions. The size of your pond will not stop growth, poor water conditions is the only thing that will stunt the growth of a koi.

NEVER ever clean your filter with tap water, unless your tap water is actually well water. You should always clean your filter with the pond water. When you clean the filter you don't want to clean too much, for example if your filter media was white when you set the filter up and is now brown you don't want to make it white again. Brown is good, just rinse it off and stick it back in. The more you clean the more good micro organisms you lose. Remember the bio filter isn't to filter out solids it's really to filter out Ammonia and nitrates so the good microbes that are growing in your bio filter are what you want to keep and they generally turn things brown.

The red worms are good for your pond. They don't do anything to the fish. The fish can eat the worms, which are the same thing as blood worms that you buy in the store. The little red worms are a good sign.

When fish are dieing and gasping you need oxygen. What most likely happened is the ammonia in the water burned the gills on your fish. You could have raised your salt level to .3% and that will protect the fish gills from ammonia levels with added slime coat and it also helps add oxygen levels in the pond.

Hopefully you get more fish for you pond, we are here to help if you have anymore questions.

Scott