Fish in QT for weeks, no issues then sudden death?

[PAXpress]

Hello everyone.
I'm a little upset. About two and a half weeks ago I got 3 Bengaii Cardinals. They were drip acclimated for an hour and a half and then moved into QT. QT is a 20 gallon long with a heater some pvc a few boring rocks and two sponge filters. Everything seemed fine, fish were eating looked healthy and didn't seem to have any problems. I was testing water params about weekly with everything looking normal. No detectable ammonia no trites ph showed somewhere around 8.1-8.3 (purple on the api test) and nitrates seemed like they were fairly low but not at zero, somewhat orangeish on the API kit.
The tank doesn't have a light but is located right below a light that we turn on every day and off before we go to sleep. It isn't super bright. What would most likely be the cause? I checked the temperatures and that seems normal. I did have the one sponge filters flow stop for about half a day but the other was running and so was the circulation pump and I somewhat promptly fixed the flow on the sponge. Yesterday I noticed one of the fish stuck to the circulation pump (its a fairly low flow circulation only about 500GPH + or - 50 or so). So I tested and everything seemed the same as above. So I did a small water change maybe about 10-20% didnt want to shock the others. SG was a bit high probably from evaporation I added a bit of top off water about 1 gallon of RO/DI to get that to more normal levels. Just a bit ago I noticed another one of the cardinals dead. I'd really like to save the last remaining one if possible. What do you guys think is happening? Nitrates? ammonia spike from filter stopping? stress? what steps should i take to try to save the last cardinal. Thanks guys.
Edit: Been feeding daily or twice daily some days. I normally watch to make sure they are all getting food. I feed frozen marine diet. Didn't really notice changes in eating but then again sometimes they would spit the food back out etc. which I thought was pretty normal behavior. Could what they are getting fed be a problem?

 

[Deinonych]

When you drip acclimated them, what was the bag salinity vs. that of the QT?

 

[PAXpress]

When you drip acclimated them, what was the bag salinity vs. that of the QT?

I believe the SG from the LFS was 1.024 qt was most likely 1.025 or 1.024. I don't remember exactly. I know it wasn't very different and I dripped for quite a while. Strange thing is they did perfectly fine for two weeks. If the SG was the cause wouldn't it have caused issues sooner than two almost three weeks later? This tank does have a glass lid to prevent too much evaporation, however I did notice a bit of evaporation had happened and SG was at about 1.026-1.027 yesterday when I noticed the first dead fish. This is when I added a gallon or so of RO/DI.
Also I do use a calibrated (with 35 PPT not RO/DI) Refractometer. So it should be accurate.

 

[Deinonych]

OK. That isn't a large enough salinity difference to have an adverse effect on the fish. It's possible some sort of disease was at play, but it's really hard to say what it was if the fish weren't exhibiting any symptoms prior to death.

 

[PAXpress]

OK. That isn't a large enough salinity difference to have an adverse effect on the fish. It's possible some sort of disease was at play, but it's really hard to say what it was if the fish weren't exhibiting any symptoms prior to death.

Would you recommend 10-20% water change and continue with QT and just hope for the best.

 

[Deinonych]

Water changes won't hurt, and definitely continue with QT and monitor closely. Any unusual behavior like reclusiveness, or scratching/twitching?

 

[PAXpress]

Water changes won't hurt, and definitely continue with QT and monitor closely. Any unusual behavior like reclusiveness, or scratching/twitching?

Timid and a bit jumpy but no scratching or twitching that I've noticed. I remember seeing them retreat behind the rocks when someone would walk by the tank or something...

 

[krullshards]

Any sign of nipping or fighting, frayed fins? From what I understand you should be able to keep a few of them and they kinda hang out together. But too many and they may fight. What's too many? I have no idea. Probably depends on the size of the tank. I was going to get a buddy for mine but was warned against it.

 

[ThRoewer]

Were those wild or tank bred?

Do you know what sex they were?
These guys are not really swarm fish - the most you can keep in a tank is a pair. Two females (and likely also two males) will fight to the death.
Females are also known to kill males if they are not willing to mate (which means at least 2 weeks of fasting for the male)

 

[PAXpress]

Let me find out if they were tank bread. As for the fighting they were all together when I bought them however their tank was much bigger than my QT. In the few weeks they were alive I never saw even the slightest hint of aggressiveness towards each other they always were swimming near each other and almost seemed comforted by it. I know this equates to essentially nothing because they could have been completely different after lights out towards each other. This is a possibility I didn't really consider (because of the behavior I saw). I knew they were aggressive towards their own kind but I did hope buying them together from a group that was obviously friendly would prevent issues. Well I did a 25% wc last night and checked on solodojo this morning and he/she is fine still so maybe that is the reason. Tests are still not showing anything different than usual with the water params. I don't know the sex of these I didn't bother to ask and the lfs guy didn't say either way. As for frayed fins not really but I did notice that the very very top fin of the two fish that are deceased did have a bit of fraying but it didn't appear to be a place were nipping would occur. It was the very top tip of their body was slightly different. I didn't think anything of it at the time but I don't think the remaining cardinal has that. Maybe it was being aggressive even at the lfs...