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New Cardinal Tetra Deaths

Hydro Homie

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Hi everyone, hope you guys are doing well.

I recently purchased 50 cardinal tetras from Aqua Huna, along with 2 electric blue rams. These guys all arrived Wednesday, March 16, 2022.

I temperature acclimated the fish for about 30 minutes before cutting open the bags and immediately plop-and-dropping them in, using a net and bucket to strain them out of the shipping water.

There were about 9 tetras DOA in the bags, but after being added to the tank, the remaining 41 seemed to be behaving normally and were even beginning to color up.

When I checked the tank the next morning, 3 of them were dead. Throughout the rest of the day, every 30 minutes or so, a lone cardinal tetra would begin to isolate itself from the rest of the shoal. It would then start to stick close to the surface, breathing rapidly while also losing most of its color, before eventually losing the ability to swim properly and spiraling down into the substrate.

I am now down to just 22 tetras, and another one is already starting to isolate itself.

As for the electric blue rams, they seemed to pair up within literally minutes of being added to the tank, and have been busy digging holes everywhere and doing other ram-my things, together of course. They have not shown any of the tetras' worrying behavior, although they have been occasionally chasing the healthy ones and nibbling on the dead ones.

Water parameters:

  • pH: ~7.4
  • nitrates: 5-10 ppm
  • hardness: ~140 ppm
  • nitrite: 0.5 ppm
  • ammonia: 0.5 ppm
  • KH/buffer: ~80 ppm
  • water temperature: 82.5 F

I am under the assumption that the presence of ammonia and nitrite are spikes due to the dead fish, and am dosing Seachem Safe directly into the aquarium, at the recommended dosage to detoxify ammonia, every 24 hours.

The tank is a 40g breeder that I am using for quarantine. It was set up about 3 weeks ago, but it uses 2 large sponge filters that have been running in another more established tank for about 4 months. The sponge filters were moved over a day before the fish arrived.

I live in Dallas, so water out of the tap is very hard. I am using a 1:1 conditioned (with Seachem Safe) tap water to RO water ratio for water changes for this particular tank. I tested the water before adding any fish, and the parameters were more or less the same minus the nitrates/nitrites/ammonia and a slightly higher pH.


What do you guys think could be the problem?

Is the water still too hard and I need to dilute the tap further?

Is everyone getting nitrite/ammonia poisoning because I am not dosing enough Seachem Safe/dosing too much Seachem Safe?

Should I have drip acclimated the cardinal tetras?

Does the tank just need to be more established to house sensitive species like cardinals for any length of time? (I am honestly surprised the rams are still alive, after everything I've heard about them being especially finicky)

Are the rams stressing the tetras to death?


I am just at a loss as to what to do right now, It's especially disheartening when the most recent reviews on Aqua Huna (the same day I got my fish, actually) are about how someone made it with 60 cardinals and no losses.

Thanks for reading.




Edited by Hydro Homie
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My LFS owner warned me that cardinals really like soft water and he strongly recommended I get an RO system before buying any. But they can acclimate slowly to harder water.

I got 10 cardinals from Aqua Huna in January. They’re all still alive. I tested the bag water before setting up the quarantine; Washington must have soft water because it was only 3 dGH. My tap is 14 dGH so I had to do a lot of diluting. They still got ich the first couple weeks. Stress from travel maybe and even if the water hardness is the same the new profile probably needs getting use to too.

Anyways, 140ppm is almost 8 dGh which is more than double what they’ve been kept at Aqua Huna. Maybe getting it softer will help them but you probably should do it gradually. Take my recommendation with a grain of salt though because I haven’t dealt with your specific situation. 

Mine are still in quarantine and I’ve been gradually raising their GH to match my display tank. Other than the ich 2 months ago they seem to be tolerant of gradual changes.

Edited by Bjorn
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On 3/17/2022 at 11:57 PM, Bjorn said:

Maybe getting it softer will help them but you probably should do it gradually.

I really don’t want to steer you wrong so I just want to reemphasize I don’t know if your GH is the main culprit. I’m just sharing from what I’ve heard and my limited experience.

I’m also really sorry to hear you had so many losses. I hope you don’t feel too disheartened for very long. Stuff like this happens to veterans too.

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What I would do is small daily water changes to dilute the ammonia and nitirte and keep adding safe to detoxify any ammonia or nitrites they probably came in in a weaken  state as the ammonia and nitirte have spiked that could be pushing the remaining cardinals over the edge  I would also add some Aquarium salt 1 table spoon for 5 gallons that will aid Gill function and add essential electrolytes at this point getting ammonia and nitirte to zero will give your fish best chance at a recovery

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  • 2 weeks later...

Thanks for the replies guys!

I ultimately went with my gut feeling and decided that it was a failure to acclimate problem, and worked with the belief that what my fish were experiencing was osmotic shock due to significantly higher hardness and the lack of acclimation.

I decided to try slowly reducing the hardness with small daily water changes with even more diluted tap water, until the hardness was about ~100ppm. My thought process was getting the fish even just a little bit closer to the water that they were used to would help them get over the osmotic shock sooner (though it is certainly possible this did the opposite and just put them in a state of shock for even longer). To facilitate this process, I did not add any aquarium salt.

I ended up losing 10 more cardinal tetras, leaving me with 12 left.

On Wednesday the 23rd (this Wednesday), I received another shipment from Aqua Huna for 30 lambchop rasboras and 30 more cardinal tetras.

In this shipment, there were 2 cardinals DOA. I placed the remaining 58 fish in a 5 gallon bucket, using Seachem Safe to detoxify their shipment water. I then drip acclimated them for 6 hours, giving them their own temperature matched heater and sponge filter for aeration.

The drip acclimation finished with about 10x the original water volume (so their water was 90% my tank water). I then poured the bucket into a net, and released the fish into the quarantine tank.

I am extremely happy to say that, with 70 fish in the quarantine tank, I have yet to lose any more.

I am beginning to believe that the difference in hardness may have just been too great for the cardinal tetras to really survive with only a plop-and-drop acclimation. But this is of course just a sample size of one. It could very well be that this new batch of cardinal tetras was just healthier from the start, and that the lambchops are simply a hardier fish in general who don't care about acclimation.

Anyways, just wanted to share what ended up happening.



Edited by Hydro Homie
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The last 3 times i've ordered cardinals - and i did exactly as you did - put the closed bag in the tank - let temp stabalize - open and remove the fishes and put them in the tank. 1 out of 3 died (0 out of 10 the first time; 1 out of 10 the 2nd time and 0 out of 10 the last time). My water is kh 3 gh 7 ph 7.1. 


Couple of comments: First the fishes were probably not doing well if 7 died in the bag - that's a lot of fishes to put in a bag and I wonder if there was some ammonia. You definitely did the right thing releasing the fishes as soon as the bag was opened. I do wonder if releasing so many fishes caused an ammonia spike. 


Tidbit the order that had the 10 cardinals also had 8 eques. The guy packaged each cory eques in its own bag but one of the bags leaked 95% of the water so the cory had no water - but it also lived. Probably because cory can breath air. I worry one morning it will die but i forgot to tag him (I didn't really forget - i just didn't want to super glue a lead weight to his fin) so if one dies i will never know which one 😉 (this was 4 weeks ago).


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