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Anna, Pet Expert/Biologist
Category: Pet
Satisfied Customers: 11067
Experience:  40 yrs.: herps, pocket pets, rabbits, poultry, dogs, horses. Biology degree. Vet assistant.
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I have a 500 gallon holding tank with 6 large koi (24") and

Customer Question

I have a 500 gallon holding tank with 6 large koi (24") and about 50 goldfish . Over the last week, I have lost 4 large koi and 2 large goldfish. We recently converted our pump to and exterior pump 4000gallons an hour and a new filter system. I have also noticed a gooey white algae on the sides of the tank and on the hose of the old pump. the fish aren't eatting much and the food sits on the surface of the water almost all day. Help!!!
Submitted: 3 years ago.
Category: Pet
Expert:  Anna replied 3 years ago.

My name is XXXXX XXXXX I'm a biologist with a special interest in fish, an dover 30 years experience with fish keeping. I would like to help you today. I'm sorry to hear of this trouble with your tank. Some additional information will be useful.

How long have you had the tank set up?

Have you tested the water? If yes, what numbers in parts per million (ppm), did you get for ammonia and nitrites?

Do any of the fish have red streaks - sort of like blood vessels- on their fins or tails?

Do the fish themselves have any of the white stuff on them?

Thank you.

Customer: replied 3 years ago.

the tank has been set up for about 4 months. I have not tested the water. Yes, there is blood streaks in tail and hemorrhage like bulges behind fins. I do not see any film on fish!

Expert:  Anna replied 3 years ago.
Thank you for getting back to me. I’m working on your answer and will post it as soon as I have it typed up.(I am not a fast typist). Please don’t respond to this post as that can lock me out of the question. I’ll be back shortly. I appreciate your patience.

Customer: replied 3 years ago.


sorry, but I am under the gun here. My fish are dying and its been 20 minutes since your last response. Again, I need help fast!!

Expert:  Anna replied 3 years ago.
Thank you for waiting. I just saw your additional post, and time is not going to make a difference. I think you are going to lose many more fish and nothing can be done. I'm sorry to be the bearer of bad news. I'll explain it all. The white stuff is not the problem. It may actually be a light-colored algae that is growing due tot he large amount of fertilizer in the water. It could also be excess slime produced by the fish due to what is happening to them. I thought you would say the fish do have red streaks. We don't even need to test the water. Your fish are suffering from severe ammonia toxicity. You are likely to lose more unless we can come up with a solution. The problem is that your tank is far too small for the fish you have in it. Someone at a pet store or pond store may have given you wrong information on how many fish you can keep. It's very common for that to happen. We think we can rely on these people, but unfortunately, we can't. I'll explain.

With excellent filtration, it is recommended to have no more than one inch of koi or goldfish per 10 gallons of water. One inch per 20 gallons is even better. That means that, at the most, you can safely keep two large koi and one goldfish in a 600 gallon tank. The minimum amount of water (with excellent filtration) you need to support 6 24" koi and 50 two inch goldfish is 2,240 gallons. At this level of overstocking, there isn't a filter made that can keep up with the waste the fish are producing. When a there are too many fish, there will be one crisis after another. You can confirm this on the following website:

The best thing you can do is find new homes for some of the fish. When that isn't possible, I usually recommend that a person do small frequent water changes, continue with good filtration, and improve oxygenation by adding more fountains or waterfalls.However, as the fish grow, there will come a time when the tank or pond simply can't support them. In your case, I don't think any of my usual suggestions are going to help because it is already at the level where they can't be supported.You'll have to remove fish or make a big pond, or fish will be dying regularly. I'm not trying to discourage you, but I believe you deserve honesty about what may be going on and what is likely to happen in the future.

The hemorrhage like bulges are exactly that, hemorrhages. Once that starts it is nearly impossible to save the fish. The ammonia causes internal damage, and it cannot be healed.You will probably lose many more fish. There is no water additive that can help. I can tell you enjoy your fish and care about them, and I feel sad to give you such bad news. I just can't lie to you and tell you to try this or that, because there is nothing you can try that will work. I'm sorry.

If you want to try to save the fish that are not yet hemorrhaging, you can provide temporary quarters by setting up large plastic wading pools in the shade so the water won't overheat. Use water that has been de-chlorinated, and follow the guideline of one inch of fish per 10 gallons of water. You can also add pond/aquarium salt (sold in pets tores) at the rate of 1 teaspoon per gallon of water. Ammonia removers, such as Ammo-lok can also be sued, but again, that si only a temporary solution. If the fish recover, you can come up with a permanent solution, such as finding new homes or putting in a pond.

If you have more questions, let me know by clicking on REPLY. I hope you'll be able to save at least some of your fish.


My goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back, I am happy to address follow-up questions. Please remember to rate my service only after you have all the information you need. Please note that you are not rating how happy you were with the information, but rather, the quality of service only. Thank you!

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