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A bad day for me and my fish - what do you think?


Active Member
Founding Member
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Oct 31, 2022
Yesterday was a very bad day for me and for my fish, one of my tanks is a community tank with different species, it was the weekly maintenance day where I do tanks' maintenance and change 50% of the water and of course use a de-chlorinator to remove any tap water chlorine; I have been doing this for almost 3 years now and I'm doing the same steps every time

To cut the story short; after adding the new water to the tank plus the de-chlorinator; the fish started to suffer something, they lost balance, some of them were not able to breath, and in just a few minutes I started losing the fish one after the other, in almost 30 minutes I lost 9 fish (3 cardinal tetras, 1 zebra danio, 2 black empire tetras, 1 cory cat, 1 otocinclus, and 1 Harylquin rasbora) - in my 3 years of the aquarium hobby and fish keeping I never faced the same issue before

I don't have currently a chlorine test kit or test strips but I think it was an excess chlorine in the water that the water company for some reason decided to add more than the usual amount of chlorine, this extra amount caused this situation, but it was very fast that I couldn't handle fast enough, at the end everything was stabilized after adding extra amounts of the de-chlorinator

After things are back to normal now, I believe there are several points to learn from this accident:

- I must keep a chlorine test kit to test the water to avoid using it in case the chlorine was more that the usual
- I am using a local made de-chlorinator which is much cheaper than the brand names, but I believe I might return back to brand names like Seachem for example
- Facing this situation caused me not to think straight, I should have moved the fish at once to another established tank
- I might go for changing 30% of the water instead of 50% to avoid a huge change in the water parameters

What else would you advice as precautions while changing the water?
Often times now that the weather is changing (raining more, cooler temps, snow melt etc), the water municipalities change there formulas of what all they add to the water we drink. Some add other chemicals not just Chlorine or Chlorimine. This is the cause of quite a number of fish deaths for me. Here in the USA every district has to publicly share this info. It is normally located on their website. You can also request via writing the water test reports for free. Note that it is just the basics. If you want say exact ppms of Ca and Mg or heck even Fe you might have to pay for that.
Now to fix this issue simply create a water storage tank like the reefing community does.
I'm sorry to see that you've gone through this. If you've been an aquarist for an extended time, something like this will unexpectedly happen.

So, I'm not sure in your location but in the USA, chloramine is being used more and more versus chlorine. It is much harder to break down and neutralize chloramine so the danger of it get through into an aquarium is much higher.

I think this is one of the areas that most don't think much about. You dump dechlorinator and move on.

Some of my suggestions are:
  • As you mentioned, make sure you know you are using a quality dechlorinator AND know how to use it if you have chloramines.
  • I'm not a fan of huge water changes because of the instability that they bring. I understand some times they are necessary but we need to know that they replace 100% of everything - the bad AND the good.
  • If you can, I would use a carbon filter to run my replacement water through. There are ways to do this DIY that are very simple and provides a nice backup. Besides, it can also remove things beyond chlorine that we don't want in the tank.
  • I use RODI so this is not an issue for me but even with my setup, I use two carbon pre filters to ensure chloramines are eliminated.
Unfortunately in my country they don't care about publicly declaring what they use to dose the water, so there is no option of getting exactly what are the chemicals, metals ... etc. inside the tap water. For me and my family we stopped using tap water in drinking for like 20 years now, we depend mainly on bottled water from reputable international companies

@BigWave I have already a 250 liter stainless steel water tank that I use mainly in emergencies when the tap water supply is closed for maintenance or for any other reason, I believe this would be a good option, it will be kept for at least 1 week before I use it but from what I read I believe Chlorine only will evaporate by time, but Chloramine and in case they dose other chemicals then I am not sure these will evaporate by time

@Art yes, I believe using an RODI unit might be an option specially that I am currently working on setting up my 4th and 5th tanks, so there will be too many fish to risk, specially that I was planning to do all the tanks' water changes on a single day, a quality dechorinator is a must I believe if I didn't go for the RODI, money wise an RODI unit would be much cheaper on the long run, didn't know about these carbon filters before, specially the DIY ones, will definitely google this out and see

@BigWave & @Art thank you very much for your suggestions, much appreciated
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Today was the weekly water change after last week incident, I decide to stop using the local de-chlorinator brand and used Seachem Safe instead, I added it to the fresh tap water in a plastic container, left it for a few minutes before adding the water into the tank ... and everything went just fine

I believe I can now make big water changes without risking the fish šŸ˜Š

Thanks @Art for mentioning the quality dechlorinator, I still can consider a RODI unit in the future, but so far Seachem Safe is a cheap and easy solution