Thank you so much for your response!
I don't have the exact nitrate ppm with me, I'm at work. It has been steady for years though. The tank, with the barbs, was established mid 2018. I've been told that a consistent level is ok.
I only did the 50% change once they were all dead already. Typically I do more like 15%-25% weekly. What do you think of that?
These fish were huge and healthy and active.
It's heartbreaking.
Thank you for any information and thoughts!
Okay, I read it that you did the water change and then they died. Big difference.
A 15%-25% weekly or even twice weekly should be fine. As stated, Dennison's do not like quick changes to their aquatic environment.
Here's the thing with nitrates. They are not totally harmless. They are less harmful than ammonia and nitrites and need to be in lower amounts to maintain that harmlessness. With long term exposure to high nitrates, nitrate gets into the fish on a cellular level and over time, causes organ damage. This is why a level under 40 PPM and ideally more closer to 0 is what's recommended. The good news is that fish can recover from nitrate " poisoning" by having good clean water so that it flushes out the fish's cells with good water.
This is from PetMD:
The Potential Dangers of Excess Nitrate Levels
Sure, you've been told so many times that nitrate is harmless. A lot of fishes can tolerate brief exposures of up to 550 ppm. Chronic exposure, on the other hand, can indeed be damaging, even at much lower levels of exposure.
Over time, at just 30 ppm, nitrate can negatively impact cell development in both fishes and invertebrates. Lethargy, poor color, poor immune system and weakened feeding response are all signs of nitrate poisoning.
Most professional aquarists contend that nitrate concentrations should never exceed 20 ppm but are much more safely maintained below 10 ppm.
Still, nitrate concentrations of just a few parts per million can lead to massive algal blooms. These may occur as either planktonic (e.g., "green water") or benthic (e.g., film or slime) blooms.
So while consistent parameters is good, consistent correct parameters is what's necessary for long term health.
Now, with all that said, when a group of the same specie all die in a tank within a day or two of each other, that usually says something in the water did not agree with them. In your case, it may have been the nitrates? May have been a change in the oxygen level in the water? May have been a dramatic temperature change? Unfortunately, to determine if any of these were the cause, you would have needed to test the water when you found the fish dead. Other than that, it's really all guesses without an actual autopsy by a professional.
Let's start tho with the actual nitrate level.