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I've lost 10 fish in 2 days


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Water Parameters:0 Nitrates  0 Nitrites 150 GH  120 KH   7.6 Ph   0 Chlorine (taken Sunday and the same on Monday)

Nothing new in the 29 gallon tank that receives weekly water changes. It has been up and running with fish since March.   On Saturday I completed a standard 30-40% water change. Its hard to know the volume because there is probably three inches or more of substrate.  Sunday morning I woke up  a dead hatchet fish. Two more, Monday morning and two were found Monday after work. The last two had a red stripe in what I could only assume was the gut.  Did a 50% water change then dosed Maracyn as directed.   Tuesday morning more dead.  Dosed Maracyn again as directed.  Came home Tuesday from work and ALL of the hatchet fish were dead and now two blue kerri tetras.  The entire tank to include all the plants and the walls of the tank were covered in this white slime. It was also coming out of the filter water intake (Seachem 35) also run an airstone and a small sponge filter I was cycling for a new tank.

I moved all of the fish out into the hospital tank.  I didn't know what to do!  Did a 100% water change in the 29 gallon.  That was all Monday. 

As of today there are only 3 tetras left out of what was 10.  There are some ember tetras, to big mystery snails and panda corys left.  I dosed the hospital tank with Maracyn.

I have two other tanks that also received water changes on Wed last week they are fine. I clean my nets in bleach, rinse and let set for several days before I use them again.


Any ideas about the white slime? Could it be the cause of the deaths? Can I dose salt with mystery snails? Do I just scrape the 29 gallon and all the plants as a loss?

Tomorrow, I am going to try and set up three species only tanks on a wing, a prayer and substrate from my two healthy tanks that are left.

Please let me hear your suggestions.  I am afraid I am going to lose all my little guys.

Thanks,  Julie


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On 9/2/2021 at 10:29 PM, Colu said:

The fact that this happened after a water change I would suspect something in your tap water did you use a dechlorinator and have you tested for ammonia also I would test your tap water

@Coluhit it on the head with his questions. If nothing was added to the tank and this happened after a water change something got in your tank during the water change through your source water, your dechlor, the tools you use to change water or even if you had some remnants of something on your hands (soaps or lotions for example.)

For your water change method do you use something like a faucet based Python to change the water, or are you doing it with a simple siphon and a bucket? Any chance any of those tools could have been contaminated with anything?

When you reintroduced water, was it tap water? Bottled? Are you on municipal water or is yours sourced from an inground well? Water quality from the tap is sadly a moving target for a lot of people. When it comes to municipal water, the companies that treat it are aiming for the water to be potable, they are not concerned with how the chemicals used interact with fish or other animals that get in it. The treatments they use change based on chemical procurement contracts, or based on local laws that require them to do something, so something might have changed. If you're on a well, you're much less likely to get impacted by these municipal water issues, but at the same time if you're near any agricultural land and they use pesticides or fertilizers, those can get into the well as rain water brings those chemicals into the ground over time.

I noticed you said that your water test params showed zero nitrate, is this common for you? Trace amounts of nitrate are fairly common, so seeing a zero is something to note. If your test kit is old or contaminated, you might want to get a new one to compare results.

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If the hospital tank is running okay, then would that still indicate a problem with the water? Was the same source water used for the hospital tank vs. the main tank?

It also sounds like you're using test strips judging by the numbers provided. With the tank collapsing like this, I'd get your hands on a master test kit if you can and see what values it turns up. I wish I had answers about the white/clear slime, but it looks like one of those things where we'd have to stand around and take a closer look at the stuff to figure it out. It's likely related in some way if it showed up at the same time as the fish started to die, but whether it's an overgrowth of something due to an imbalance or something else, I have no idea.

EDIT to add: I'm so sorry for your losses, by the way. It's heartbreaking and frustrating, and I hope you're able to get to the bottom of what happened.

Edited by laritheloud
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Thank you all,

Yes, I use the dechlorinator made by Fritz and did that night. 

0 ammonia in water straight from the tap, the hospital tank and the death tank that now has been running for greater than 24 hours with new water.

0 chlorine in any of the three conditions

Tap shows 0 nitrate and nitrite. Lower KH and GH which I would expect since I bulk the tanks with crushed coral.

I usually have some nitrate but this was taken pretty soon after a big water change and gravel vacuuming. 

I am on city water. I do use a python it was cleaned after the other tanks were cleaned on Wed of that week. So it sat for several days. Both of those tanks are fine.

Nothing is transferred between the tanks that isn't cleaned first.

The fact that it was hatchet fish that died first made me think that it was on the surface of the water or at upper part of the water column. 

I'll go and get an API kit tomorrow.

Thank you for the thoughtful answers and condolences.  Maybe it was something fluky with the water that night. 

I'll test with the API kit and if it shows anything different update the post.


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The white slime is interesting. Do you have any more pictures of it? Also just want to make sure you hadn't used any sunscreen on your arms that day as sunscreen is lethal to fish. Just a couple things off the top of my head. It's an odd case I hope we figure it out.

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