# Fish dying overnight



## policebox (Apr 14, 2012)

In the past two weeks I've lost about half of my stock. I wake up in the morning and there's generally one or two bodies sucked to the filter intake screen. I'm truly at a loss as to what is causing these fish to die. There are no apparent signs of disease on the bodies. There is no sign of disease on the live fish. I don't see any signs of stress either. They generally seem content and feed well before I go to sleep, but when I wake up there are causalities.

Setup is a 55 gallon planted freshwater tropical community tank.
pH: 7.2-7.6
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 10 ppm
GH: 30 ppm
KH: 80 ppm
Temperature: 76º F

Species present:
11 Peppered Cory Catfish
11 Zebra Danio (from original 12)
8 Serpae Tetra (from original 13)
2 Black Neon Tetra (from original 13)
5 Otocinclus (from original 7)
9 Cherry Barbs

I feed a variety of flake, frozen brine shrimp, sinking algae wafers and shrimp pellets.

The deaths began the day after I added the Zebra Danios to the main tank. Prior to adding them, they were quarantined for 10 days, with no signs of illness. I haven't noticed any species harassing the others. Although the water tested fine, I did an emergency 20% water change today after finding three more dead fish this morning.

Any thoughts about what else I could check to try to get to the bottom of this?


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## navigator black (Jan 3, 2012)

It has the pattern of oodinium, velvet. Look for a brassy, coppery dusting behind the head or on the fins of the fish.
Worst case? The zebras brought a bacterial or viral disease that they have partial immunity to (you've only lost one) but the established fish (black neons, serpae) have no immunity to. I'm afraid that's what I suspect, given the pattern of deaths among the fish. Only the Asian cherry barbs (like the Asian danios) are unaffected, among the schooling fish. It doesn't explain why the paleatus are unaffected, but your losses are mainly South American fish.
I've had tanks of wild fish to which I introduced domestic fish, and had the wilds wipe completely, as the domestics swam by happily...


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## policebox (Apr 14, 2012)

Thank you for your reply. That's an interesting theory you have. It would be consistent with the zebra danios making it through quarantine but then killing off other species. I purchased them at Petco, so I do not know whether these specimens were wild-caught or farmed.

I looked that the live fish just a few minutes ago and also examined the bodies of the dead ones, but do not see any signs of velvet, copper spots, ich, dropsy, or even stress. I’ve never noticed any fish hiding in a corner, or exhibiting a swim bladder problem, or clinching their gills/fins. Each day they all seem to be contently swimming around, then the next morning something is dead.

The only thing different during the night from the day is this. I run the yeast-method DIY CO2 injection since it is a planted tank. At night the CO2 is discontinued and an air pump hooked up to a bubble wand is used to oxygenate the water. I discount the deaths being caused by asphyxia since I’ve never seen any fish at the tank gasping for air in the morning and don’t think the addition of CO2 is killing them. Since the water here is pretty hard, I also can’t imagine there being a big swing in pH during the night. I’ve tested the water at several times during the day, using more than one test kit. I’ve also taken water samples to the pet store for testing to make sure my test kits are accurate; all the tests come back pretty much the same.

At this point I’m not sure what to do. I don’t like the thoughts of treating the tank with medicine, since I don’t know what I should treat for since there are no external signs of disease, I don’t want to destroy the biological filtration, the tank is planted, and the tank also has many Melanoides tuberculata snails. I suppose I could put the danios back into the quarantine tank, but I’m not sure what that would accomplish in the long-term.

For now I did a 20% water change and put the carbon back in the filter. I generally run without carbon in the filter so as not to remove the plant fertilizer. It’s never been a problem since I use R/O water and make 10% water changes weekly.


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## Manafel (Sep 4, 2011)

If its bacterial or parasitic, your whole tank is infected, so there is no point in putting the new fish back into QT. To me it sounds bacterial. Grab some antibiotics like amoxicillin. I had an outbreak of columnaris(nasty stuff) in my 75 and I had no choice but to dose the whole tank. Lower the temp. A bit to slow down the life cycle of the bacteria.


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## navigator black (Jan 3, 2012)

I'll disagree with manafel, something that is rare with his/her postings. Bacteria are either gram negative or gram positive, and you don't know which you have. So you don't know what antibiotic to use. Every misuse of an antibiotic is a potential aid to new antibiotic resistant strains.
It could also be viral, in which case all antibiotics are wasted.
We have a ban on aquarium antibiotics where I live, and when it came in, I was really worried. I lose fewer fish to disease now than I lost to treatments when they were readily available. It has actually been a surprisingly positive thing, since we tended to throw mostly expired pet shop pills at all kinds of inappropriate things. Amoxicillin is brilliant for columnaris, but what pathogen is in your tank?
You will probably never know.


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## Manafel (Sep 4, 2011)

That is true, but amoxicillin helps with both gram negative and gram positive bacteria. I really only suggested antibacterials because with a large tank, it would be expensive to treat with normal medications. Who knows though. It could be something other than bacterial. If there are no external signs, the only way to tell is by examining samples.


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## snail (Aug 6, 2010)

I tend to agree about not dosing meds for unidentified problems. Of course they could help but are just as likely to cause more problems, unless you know they work for what you are dealing with. 

My first thought when reading your first post was to to ask if you dose CO2. I get what you are saying about no gasping etc but sudden, unexplained deaths at night with no sign of stress or illness fits so well. It could be something like the fish die at night not long after you turn off the lights but the CO2 is gone by the morning. You might consider testing for CO2, or an easier solution would be to disconnect the CO2 for a couple of days and see if the deaths stop, if they don't you know for sure it was nothing to do with that.


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## giddetm (Apr 30, 2011)

I agree with snail,I have a 150 tall that is heavily planted and heavily stocked. I was using the yeast CO2 bottles and was having the same problem you are having. I stopped the co2 all together and put a bubble wand in the tank on a timer that goes from 2AM- 5AM. The fish stopped dying,the plants are growing fine without the yeast. I add a little liquid fert and chelated iron when I think of it. If you have alot of plants in your tank get rid of the CO2 and run the air pump at night for a weekor so and let us know what happens.


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