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Since moving house 8 weeks ago iv'e inherited a fish pond containing around 20 fish which is 270cm in length, 130cm wide, and 35cm deep in the middle. It uses a Hozelock Cyprio Ecopower Plus 5000 Filter which has a maximum flow rate of 2250 LPH.

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I have never owned fish before, so reading the instructions on the back of the fish food the previous owners left behind it said to feed them 3 times a day, as much as they can eat in a few minutes. I felt this was a lot, because they only manage one cup in a few minutes and talking to others it would seem to be. So i have been feeding them once a day, usually around 6pm. They have one cup full of this(the cup is in the picture)...

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This morning 5 of the fish are dead and i am concerned that there is something wrong with the pond water, pond itself, or the feeding pattern.

At least one of the fish in this picture looks swollen, or is that normal?

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I looked in the filter and can see lots of insects that look like black maggots inside moving around (those black things on the foam)....

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Could there be a problem with the filter? should there be insects/life inside?

The fish that are alive don't appear to be interested in the food today (you can see it floating on top of the water in the first picture).

Also is the pond too busy? there are quite a lot of lilly pads, and the plant towards the bottom end on the right has grown significantly recently. Could the fish be getting trapped or stuck in one area of the pond?

Behaviour wise, they seem to congregate by the water inlet, gulping at it. And often come to the surface and just gulp, ignoring the food. One of them is constantly swimming on its side in a shallow area, which is unusual.

Any advice greatly appreciated, as i am concerned that more will die if there is a problem with environment they are living in.

To summarize the key concerns...

Do one or more of the dead fish look swollen? What kind of fish are they? Should the filter have life/insects in it? and is it dirty looking? Is the pond overstocked? Should they be gulping at the surface? Is the feeding pattern ok?

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    Gulping could be a sign of not enough oxygen in the water. This answer might help you work out if the pond is overstocked. Especially if the filter is clogged, they could be using up the oxygen faster than it's getting replenished. – starsplusplus Jun 8 '14 at 16:46
  • Thanks. From the picture, would you say the filter is clogged? – Baldy Jun 8 '14 at 16:50
  • @Baldy, I can't tell from the picture, but if you need to ask, it probably needs to be cleaned/changed. – James Jenkins Jun 8 '14 at 18:21
  • There's probably some sort of removeable part inside that holds the filter medium. Have you tried removing it to see if it looks clogged? – starsplusplus Jun 8 '14 at 20:38
  • ive been advised to leave the filter as-is until ive got the ammonia levels down. – Baldy Jun 10 '14 at 12:52
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After seeking advice I performed an ammonia test on the pond water and it came back at 8, which is the highest reading on the chart.

This means that the water is poisonous to the fish. It turns out that I have been feeding them WAY too much - 1 cup per day, when they should have around a tablespoon.

I performed a 60% water change yesterday, and the ammonia level is at 6 this morning. I will do a 75% water change tonight also and see how things settle down after that.

I have also been advised to not feed them for a week.

UPDATE: Did another water change (75%) and also removed the lily plant, which turned out to be nearly 7ft long.

The fish now have a lot more room and the ammonia level has dropped to somewhere between 4 and 6.

I shall repeat this process every other day until i see acceptable ammonia levels.

UPDATE: Did a full water change, and put the fish in a large kids pool temporarily. I used the pond water in the temp pool, and they didnt seem too stressed.

There was a lot of sediment in the bottom of the pond, and stones too. Removed all these but did not scrub the pond.

I also carefully cleaned the filter foam in the pond water (old), this removed the dirt and sediment but didnt destroy the good bacteria (which needs to be retained). I also cleaned out the bottom of the filter box, and removed the thick brown smelly gloop that was in there. The final job was to dismantle the 'in pond' filter, clean it, and put it back together again.

This took around 3 hours, and i added 10 buckets (130 litres) of the old pond water back in, so the shock of the new water wasnt too much. The pond holds around 1000 litres in all.

2 days later the water is very clear, the filter works well and ammonia levels are down to 1, which is a lot better than 8.

  • I'm not as familiar with freshwater, but in the saltwater world too frequent water changes will remove beneficial bacteria that dispel nitrogen compounds (ammonium, ammonia, nitrate, nitrite); the rule of thumb being no more than %30 every 3 days. Your plants are also crucial in this process as well as oxygenating/removing CO2 the water. Finally, I don't know how frequently they're found in ponds, but a protein skimmer is a device that helps remove solid waste (including excess food) from the water, before they nitrogen is dissolved into the water. – Gary Jun 12 '14 at 17:44
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    I wouldn't even feed them every day. Maybe 2 to 3 times a week unless they look malnourished. Need to give it a few more weeks for ammonia and then nitrite to go away completely, but at 1 cup of food, it seems very obvious that was your problem. – Jestep Jun 17 '14 at 21:38
  • Yep. The best method to determine how much as much as they can eat in a few minutes is, is to give them what you expect will be far less than they what can eat, watch them eat it until it's gone, and see how much of the 3 minutes has passed. Then add more food and keep watching. If your first bit of food was consumed in 60 seconds or less you could confidently increase the amount you put in the second time; if the first lasted more than 90 seconds you'd add less than that the second time. After a few feedings you'll have worked out the amount and be able to put it all in at once. – Dan Henderson Sep 18 '15 at 15:24
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This looks like a form of Dropsy. Dropsy is a generic term for edema in fish often caused by bacterial infection.

Fish get sick for all kinds of reasons, stress, oxygen level, water quality, infection and more.

So in this case it sounds like you have bad water with some bug.

We used to use Potassium Permanganate in tanks where fish are showing signs of illness. It has disinfectant properties. (See link) http://www.theaquariumwiki.com/Potassium_permanganate

What I would do is move an ill-looking fish into a quarantine tank (with the same pond water) and treat it with Potassium Permanganate, slowly (a small pinch at a time). Too much will also harm it. Check for any improvements.

You could also check the pH of the water. Certain pH levels are and temperatures make for favourable environments for bacteria to flourish.

If you do find that the Potassium Permanganate does help- drain about half the pond's water, treat it, and then introduce fresh water.

On feeding the fish too much: I was taught that fish will eat themselves to death. So the measure that I was taught as a kid was as much the fish can eat in 1 minute. Or, if you feed regularly- give as much as the fish's eye at a time.

Hope that helps!

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    The KMnO₄ will turn the water a dramatic purple or pink. So you will sort of have an slight indicator of how low it stays in your tank. This is where I would go "Slowly". If you cannot for a fact identify what the fish is suffering from it's best put your first pinch in and watch and wait to see any improvement or deterioration. Illnesses such as Fin-Rot and White Spot can be treated more persistently. BUT I would still advise you to consult someone who deals in fish for a living. There might be some 'medicine' specially developed for fish. – FredFury Jun 10 '14 at 21:55
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I would add a small amount, say a quarter of a cup of evaporated sea salt. After 50/0 water change twice a year. It stops parasites and the fish love it, it keeps their stress level down. But the plants don't like it so not too much.
I have had the same 5 koi for 18 years and 10 gold fish in the same pond, about 5000 liter with no loss or sickness or bugs .. I feed them 3 sauce caps of food in the morning and 3 in the afternoon in summer, and during winter only one feed in the morning.
Clean the filter when the water starts to slow down on the flow out and make sure the water spout into the pond is air rating the pond. Don't just lay the return in the water. And your fish will love you.
Don't just keep adding salt without some water change as it never goes away and can build up salinity.

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