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Fish developing red patches, bloating, and expiring suddenly


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Longtime lurker, so of course my first post is with a problem.

Tank is a 29 gallon planted with a female koi betta, 13 rummynose tetras, one (remaining) black neon tetra, two (surviving) Julii cories, and red cherry shrimp. I am running an airstone.

  • pH = 7.6
  • Nitrates = 25
  • Hardness = 300ish
  • Nitrite = 0
  • Ammonia = 0
  • KH/Buffer = 120
  • Water Temperature = 78 and trying to bring down

In the past day I have suddenly lost a rummynose, then a black neon I have had for about 5 years, and then another rummynose. Their bodies show some patches and bloating (see photos).

The remaining black neon has red gills (see photo) and a couple other rummynose have red patches. They seem to be having some trouble breathing and are not swimming with the school. 

Yesterday I did a 10% water change and fertilized. I may have overdosed with iron since I used Easy Green and then added another squirt of liquid iron (Ultum Nature Systems) to keep the scarlet temple plants happy.

Today I've done a 20% water change, lowered the heater temp, and turned down the lights to "cloudy day" setting to try to reduce stress.

What might be going on and what are recommended next steps?

Backstory: One June 15 I brought home 6 Julii corys from my LFS. One died almost immediately and another two in the next two days. They seemed to have trouble swimming and kept sitting in the plants at the top of the tank. I lost another one on the 22nd. The remaining two seem to be doing fine. 

Thanks in advance for any help!

IMG_6407 2.jpeg




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On 6/26/2023 at 9:05 PM, Ron Uni said:

Do you know if you have ammonia?

I believe it is zero. This is a tank that's been set up for a few years and is testing at zero nitrites. I use the Fritz water conditioner during water changes. 

This evening the remaining fish all seem to be swimming around like normal, except the last black neon. 

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Could be bacterial Gill disease bacterial infections can cause bloating looking at the picture I would treat with a broad spectrum antibiotic treatment such as kanaplex or maracyn2 I would also add a small amount of aquarium salt 1 table spoon for 6 gallons at that level it shouldn't harm your plants and will aid Gill function and add essential electrolytes give an update after your first course of kanaplex or maracyn2 if your seeing no improvement @laffer

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Hi and thanks for the suggestions.

After treatment, everyone seems to be doing okay. The tetras are schooling, the corys are zooming around, the betta is keeping an eye on everyone, and at least one female cherry shrimp was sending out pheromones this morning. 

Thanks again!

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@laffer given the severity, I'd definitely follow up with a second round.  It's a good sign that they are getting better though! 🙂

Given the temp, add in some aeration if possible as well.  Try to float some bottles that arr 3/4 full of frozen water during the hotter days.  Definitely verify that heater as well! (check the temp of it, they are extremely common to get stuck running very hot).

Edited by nabokovfan87
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Thanks @nabokovfan87. I am running an airstone and the filter disturbs the surface with a strong flow.

The temp has remained steady at 77 degrees. 

I went ahead and did a second round of treatment, especially since my female betta has turned kind of lethargic. I appreciate the recommendation!

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