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Please help me identify this mystery disease.

A mystery disease has killed all the livebearers in my tank (1 molly, 6 guppies), but my neon tetras are completely unaffected.

Most noticeable symptom are blood-red marks on their bodies (pictured). First, they hover at the water’s surface, and then right before they die they hide in dark areas and develop slight pineconing. They die 1-2 days after the mark(s) appear.

The first time this happened, I waited 2-3 weeks before adding new guppies to my tank. In 3 weeks, the new guppies died with the same symptoms. Could something be “living” in my tank? Could the tetras be asymptomatic carriers?

I’d add more guppies (my favorite fish) but it would just be inhumane at this point if they’ll just die the same way.

I’ve tried treating with Aquarium Salt, Tetra Lifeguard, API General Cure, API Erythromycin, API Fin and Body Cure, and API Super Ick Cure.

The only disease I’ve heard of causing red marks was bacterial/viral hemorrhagic septicemia, but photos online don’t look exactly like mine.

Stocking and Parameters:

20-gallon long with 4 amano shrimp, 10 neon tetras, and (formerly) 2-4 guppies. Some pest snails.

Ammonia: 0.25 (it’s never been 0 even after large water changes, so I think it’s a false positive.) My tap water also tests 0.25.

Nitrite: 0

Nitrate: 0 (tank is moderately planted, pictured)

PH: 7.5






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I can't identify the illness, but I can help you troubleshoot.


Are the fish from the same store each time? A lot of stores will bring in livebearers who were raised in brackish water, and they crash without as many minerals in the water.
What is the hardness in the tank?

This would be a great time to try a quarantine tank if you have one and see if it's isolated to that aquarium or not.

How long has this aquarium been setup? I'd also try a teaspoon of marine salt when doing water changes to help buffer minerals a bit.

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