Why has my fishpond gone toxic?

[Ormirian]

We have 2 ponds in our garden. One small one near the house - about 3ft by 4ft and about 2 feet deep. And a big one at the bottom of the garden. Both have filters and pumps. We bought some small goldfish for the small pond and some koi carp for the big one in September after we moved into the house. We have a few dead ones in the small pond and some of the ones in the big pond seem to have disappeared.

At the weekend I took some water samples to the pond shop. Big pond fine. Small pond too alkaline and very high in phospates. After hearing this I rescued the remaining fish from the small one and put them in the big one - found 2 more dead bodies . The ones that were still alive looked quite sick but seemed to have recovered now.

Anyone know how this could have happened? I did treat both ponds with a blanket weed killer but chap in shop didn't think that was the problem. He suggested run-off from concrete slabs (nope) and fertilisers (none). So does anyone have any ideas? I have resigned myself to no more fish in it - we are gonig to plant it with water lillies and irises and leave it to the wildlife but I still need to get the balance right.

Hmm, run-off and fertilisers would have been my first guess too! Is there no run off at all in your garden or is there any way water other than rain water can get in? Have you topped up with your hosepipe recently? What is the small pond made of? Have you put any rocks in there recently or could some have fallen in? The pH must be pretty high for it to affect goldfish, did the man in the shop tell you what it was?

No he didn't make any suggestions apart from the one's I mentioned. It has been topped up with the hose recently - but so has the other one. Small pond has a thick liner with no rocks in the water but I guess i can't rule out some run-off from somewhere. No fertilisers in use in our garden at all. I did wonder if overfeeding might have been the issue but surely it would have to be very serious to make that much difference. The PH was low (high?) enough to be be deep purple.

And clearly something must have changed as it was fine when we moved in. Something new has happened but I don't know what.

If you live near farmland then you often get phosphate problems in ponds, mine is full of blanket weed . Fish food does contain phosphate but yes, you'd have to overfeed by a really serious amount to kill fish.

What test kit are you using for the pH?

Did the man in the shop also test for ammonia and nitrIte? Has the filter been switched off recently?

Yeah blanket weed is a real problem and the stuff I used didn't really work. We are in a town but quite near farmland - don't think it can be an issue. We switched off the pump when the pond froze as we thought it would do it some damage but it's been on again for ages now.

I think they tested for everything TBH but I am not sure. All they mentioned was the PH and the phosphates.

I am at a loss TBH.

Just seen that one of the fish I rescued has died

I feel like a murderer..... all those lovely fish gone and I don't know what we have done.

I am beginning to wonder about the overfeeding now.

Kids feed them and no matter how many times I tell them they still seem to think it's OK to chuck a bit extra in. We have got through a large tub in a few weeks now I come to think of it.

I will have to ban feeding by anyone but me from now on.

It's tough to say what happened so don't blame yourself. It's a lot harder to control the water quality in ponds because you don't control everything that goes in and you're not doing water changes every week, that's why the stocking levels for ponds are so much lower than an aquarium.

Banning the kids from feeding is a good idea! Or if they are still keen to do it then you could put the right amount of food in a little ziplock bag and just let them have that instead of the whole pot of food.