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Neon dying 1 by 1 no indication of disease and water par fine


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Hi Everyone, 

New 40 breeder here. I finished 1 month cycling the water, already had/have 3 Nerites and ghost shrimps since 2 weeks.

ph of 7.5 but going down to 6.8 when the CO2 is in its peak.

Ammonia about 0.25.    chlorine is 0.0      Nitrate and nitrite are 0.0

hardness was very very high (300GH - I am from Florida) and I have used those pillows that reduce the hardness and now it is about     25GH

About 40KH

Temp 75oF

2 days ago I put 10 neons (slowly acclimatizing them) and after 4 to 5 hours they started to dye 1 by 1.

I can’t see anything wrong in their bodies.

They start to spill and swim erratic and dye.

do you have some clue that could help me ?

thank you VERY MUCH

Edited by Marcelo
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It’s hard to diagnose any fish illness that doesn’t physically show up as a huge red flag typically, in this scenario it could be a number of things: let me preface this by saying I’m no expert, however if I were in your shoes, given the info provided this is where I would start:

-ammonia, some fish are much more sensitive to it than others so even the low number could potentially be effecting them slightly 

- water temperature 

- that ph swing with the CO2 injection could also be a culprit 

- oxygen level in the water 

- could they perhaps have been purchased sick? Improperly quarantined and one of the things listed above expedited there condition?

I theorize that due to the speed they are dying there is an underlying water quality issue in the tank or something in the tank causing this, I would master test kit it or look up your water via your water company to see if there are any pollutants or chemicals you wouldn’t think to check for


take a water sample and test it yourself or have a local pet store test it for u to get a second set of eyes on your numbers

seachem prime 

make sure your heater if in use is functioning properly and that the temp is stable and within parameters they like or tolerate 

it’s hard to have too much oxygen in my experience, make sure there is enough for them 

the erratic swimming suggests something is in that water they aren’t agreeing with, perhaps even consider seeing how they do in your water without manufacturing it to there specifications there are a lot of neon tetras in Florida water, perhaps it could be something as simple as attempting to shift your ph gh kh with pillows etc is effecting them more negatively then whatever the constant would be without doing so.

quarantine + med trio pretty much everything bc the illness could already be there and the acclimation itself causes enough stress to bring it out

sorry to ramble so long I just hope something in this helps with the issue, I will continue brainstorming, please keep us updated if anything listed works…


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Neons are black water fish so they really need clean bacteria free water. 

Check this out:

 Per the book “Fundamentals of Ornamental Fish Health”, Helen Roberts, 2010, Page 181:

“The most common bacteria in this group that cause disease in fish are Streptococcus spp.; other Gram-positive genera that are closely related to Streptococcus and cause disease in fish include Lactococcus , Enterococcus , and Vagococcus . Neurological signs are extremely common in fish with streptococcal infections, and abnormal swimming behavior such as spiraling or spinning is often reported (Yanong and Francis-Floyd, 2006). High mortality may also occur.

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On 6/19/2022 at 8:53 PM, Marcelo said:

@Coluthis could be the reason. Despite by asking the store how is the water they have, is sphere something I should do in the future ? Do a slower acclimation ? 

Frist I would try and keep a more stable pH the 0.7 function in your pH caused by your CO2 wont help fish with osmotic shock and add an extra air stone and a small amount of aquarium salt 1 table spoon for 5 gallons you can drip acclimate new   fish for a couple of hours if the water parameters are vastly different to your own 

Edited by Colu
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