Can anyone help please.....my koi and goldfish are dieing day by day, not all at once but i have lost a lot of them over the last few weeks. One or two will be found floating amongst the lillys in the morning.

The pond size is 11ft length x 8ft width x 8ft depth. I have a two filters working 1 x Bioforce 12000uvc and 1 x Jebao uv filter 15000 both with UV lights inside working and an air stone in the pond. The ph level is 7.5, no nitrate, nitrite nor ammonia. The pond has had a 50% water change, the algie is non existant, no white spot on fish, they are feeding ok.

Any ideas or suggestions would be helpfull, thanks

If water quality is fine (as appears) likely to be either illness or lack of 02 I would think, given the warm spell recently.

Do you have anything disturbing water surface, fountains etc?

Any other signs on dead fish that would indicate viral/bacterial issues, i.e. fluff, sores, red veins or the like?

thanks for the response.....water fall is off, there is no evidence of infections, red vein or any evidence the fish are dead untill they are floating. Their scales are all normal, and there is an 8 inch air stone in the pond, there is the return from the filters going into the pond, the only still part of the pond is at one side where the liily pads are.....depth of pond is 2.5 ft deep not 8ft as above...lol.

Its a difficult one as they all feed well, spawning, splashing, surfacing ....just a bit stuck now and know im out of ideas.

Have you noticed if any of the dead fish have mouths wide open and gills flared? Also do they always die during the hours of darkness?

hi, some do have their mouths opened and gills not flared, but others don`t. They do tend to die during the night or seem very ill during the early hours then die by the afternoon. The air stone is on all night and filters running just in case the oxygen level drops.

The reason I asked those questions is because the open mouth, flared gills are classic signs of death from hypoxia and this mostly occurs at night.

Has there been an algae bloom followed by a die off? The recent hot weather and bright sunshine has caused an algae problem even in ponds with uv's.

Even with an airstone if more oxygen is consumed than dissolved it obviously causes the O2 level to drop, if it drops below 5ppm the fish are in danger.

From what you have told us I think this is the most likely cause of the deaths.

So far this morning no dead fish, there is no eveidence of an algae bloom but i do have some algae starting to develope in the pond. If i change out the water, clean down the pond and re set everything could this help?
How can i get around the oxygen issue to prevent the O2 level dropping?

Your water quality seems fine so making a big change won't really help. The best thing to prevent this is to increase the overall circulation of the water around the pond, increase surface agitation - i.e. could you raise the pumps outlet well above the surface and allow it to fall through air before hitting the pond. Cut back on any excessive plant growth. Plants give off oxygen during the day but the use oxygen at night.

Thank you again, I will raise the pump and run the other pipe around the outside of the pond so water falls from a better height, hopefully this will create more oxygen going in at night. I do have another pump i could add to the other end of the pond to help with circulation and move the water more. I will trim back some of the lillies as you have advised and fingers crossed from there. Again thanks for the help.

After all the changes im still loosing fish, now this could be ones ive lost over the few days that have now surfaced, and im noticing scales missing on one or two, and white gills on one. I have decided to empty the pond, clean it down completly, i have the fish in a temp pond for now and not lost any so far. I will re stock the pond after cleaning it and setting the water again. Hopefully with the weather changing and cooling down a bit, things may go back to normal.
The fish still only die in ones or twos but over a couple of days or in the afternoon i notice one or two floating.