Cardinal down, one gilling heavily

[aflux]

I woke up to a pretty rough scene - one of my bengai cardinals was already gone, and another was struggling, gilling heavily in the corner. Everything seemed normal last night, all three were eating and swimming fine.

The rest of the tank's inhabitants - 2 clowns, diamond goby, and a bunch of coflakes - seem to be doing alright. The third cardinal appears to be okay for now.

I only had time to check the temp and SG, which were 79F and 1.026. I know it wasn't ich, I've dealt with that before and it looks different.

I'm guessing it'll be tough to pinpoint the cause without the amm/ite/ate readings, but since nothing's changed recently, I'll assume those are all zero for now.

One thing that has changed recently is that I've been running one of my 250W 15k metal halides for 2 hours, twice a day, trying to get my open brain to grow some tentacles. Any ideas what might've happened?

[talonix]

I'm wondering if you're using any air fresheners or sprays near the tank as these can harm your fish. I'd really need to know your parameters to give any advice - without the test results, it's tough to say what happened. It's possible there was an ammonia surge or pH crash overnight, but I'd need to know more about your tank conditions to make any suggestions.

[aflux]

Thanks Scott, I figured as much without the test results. I appreciate your honesty.

We don't really use plug-in air fresheners, but we do burn candles in a different room. I'm not sure if that could have caused any issues, but I'll definitely look into it.

If it was a drastic pH change or a spike in ammonia, I would have thought the coflakes would be the ones to go down first. They're usually the canary in the coal mine, so to speak. I'll make sure to test the water as soon as I get home and see if I can find any clues.

[aflux]

Came home to more bad news - the other two cardinals are gone. Still don't get why only they died, all the coflakes are thriving and the clowns and diamond goby look fine.

Ran some tests, and here are the results:
Amm: 0
Nitrite: 0
Nitrate: 0
pH: 8.3
SG: 1.026
Temp: 79F

I'm starting to think cardinals are just really prone to disease or something. They were eating fine - had them on a mix of freeze-dried brine, mysis, and flake. Weird that it was just the three of them.

[creebo]

I'm going to take a wild guess here, but maybe the aeration in your tank isn't sufficient, and that's why the cardinals are the ones that died. I've heard that cardinals are pretty sensitive to low oxygen levels. I had a friend who had a similar issue with his green chromises, they all died off while the rest of his tank inhabitants were fine, and it turned out to be an aeration problem.

[talonix]

Head down to your LFS and pick up a polyfilter, it's a good idea just in case something's contaminated the tank. Cardinals are known to be a bit aggressive with each other, usually they'll fight till there's a pair left, and even then they might still go at each other. Check their fins for any damage, that's usually a sign of them fighting.