Most of my fish died, any educated guesses

[Onefish-twofish]

I went away for six days to visit other family, and had family take care of my tank. All they had to do was feed, as I did my weekly water change before I left. My tank had 11 cardinals 15 neon tetras; 55 gl long. some of these were added three days prior to my leaving after a week in quarantine, and being treated as usual. the ones in the tank before i added the new group were about a month in the tank and very healthy. All fish were healthy when I left and when I got back I had one cardinal and two tetras. the family says they lost their color, starting swimming slow and died. the three I have left are very healthy and colorful. Honestly I have no idea what has killed my fish. The nitrates were normal, all other water parameters normal. I treat for five days in quarantine with an antifungal, anti bacterial and all around parasite, medication then two more days watching for sickness.

[Strider199]

Cardinals are very delicate but neons usually take a little more to die off.

Was there a temperature change of more than 6 degrees while you were away?

I've seen Cardinal tanks at the lfs crash for no apparent reason. One day all are healthy looking and swimming nicely, the next day only a few are left with little red colouring on them.

If your parameters weren't ok I would have said an ammonia or nitrite spike.

[Brhino]

normally with relatives feeding I'd guess overfeeding, but if your water parameters are normal then that's not it. The only other thing that jumps out at me is that one week is a pretty minimal amount of time to be in QT. It's possible the new fish you brought in were carrying something but didn't show signs during that week. That seems like sort of a long shot, though...

Just a few other ideas off the top of my head:
How long before you left did you do the water change? Any chance you forgot to add decholorinator or something and the fish started dying off quickly but you were already gone? (this would probably have showed up as an ammonia spike though)
Any chance the tank went unheated for some length of time due to a power outage or something?
Are you sure your relatives didn't decide to surprise by getting you an Oscar for christmas and adding it to the tank while you were away?
Do you know if the fish died all at once, or over the 6 days you were gone? Any chance they did anything else to the tank without telling you?

[Goes to 11!]

I will only say that 1 weeks time is not an effectively lengthy quarantine And that tripling down on meds for fish that didn't obviously need it was possibly more stressful than helpful.

Please define 'Normal' using actual numbers wrt your water parameters. Filtration?

[Lady Hobbs]

You also added fish just before you left. I would expect you may have had an ammonia spike or ick got the rest of them. Those fish come from the wild and also don't do well with medications......especially medications that were probably not needed.

[Onefish-twofish]

thanks Strider, it's good to know that sometimes this stuff happens. I just really hate losing any fish for any reason.

after prying on the fams they told me that it started four days ago, and it happened pretty fast, with five or six at a time. The reason for adding fish and then leaving, was basically one very awesome moment of stupidity, I wasn't thinking two weeks ahead

Yes I know that 1 week is not sufficient enough time by most opinions, but 1 week is better than none. using multiple medicines, has been recommended to me, which is why I use it. I feel that after being treated for 5 days and watched for two more is enough time; I should say that I "feel", more than a week in a qt is detrimental givin that the fish receive the same treatment at the LFS store(a very reputable one I might add)then again in my tank with low lighting and no substrate. The LFS I use replaces their stock weekly, because they do a lot of business.

my water parameters are ammonia .25, nitrite 0, nitrate 5ppm, PH read 7.6 but I think that test may be faulty because the high range PH test reads 7.4, and I've been adding Ph neutral to lower it from its original 8. and yes I know thats high for tetras, but not enough to outright kill the whole school. I don't think it was an ammonia spike because the tank is very understocked, and has been established for some time. Also the tank is well planted, so my nitrates stay decent. I don't think it was ick either, i was around three days before to watch them, and they looked fine, so it was a total of six or seven days before symptoms were noticed and death occured. the ones that survived seem unaffected by ick; but I could be wrong.

I don't mean to offend anybody or be self righteous or snide,I apologize if I've offended you. I would just like to know where I went wrong or if I did. Lady Hobbs from what I gather you keep cardinals, could you describe your QT process and medications you do or dont use?

[Strider199]

"ammonia .25" Is your test kit old? There should be no ammonia in a stable cycled tank. .25 isn't a killer but it can go up quick without a large water change. This reading intrigues me.
If your test kit is old you could have expired chemicals which will give you a misleading reading. But also in a cycled tank which is stable you should see 0.00 ammonia when you test.
Cardinals, neons, and other smaller tetras dont take well to ammonia.

Was this reading after a water change or before?

[Onefish-twofish]

Sorry, I'm Partially color blind, especially in shades of blue or green, and I always go heavy on my readings rather than lighter, just to be honest and make sure everything is on the safer side. so I'm pretty sure there is very little to no ammonia,especially given that there is only two other fish in the tank besides the aforementioned tetras.

[Honey Badger 1]

Insufficient time in QT, using stressful medications, .25ppm ammonia, and pH swings all worked together to kill your fish.

[Onefish-twofish]

Insufficient time in QT, using stressful medications, .25ppm ammonia, and pH swings all worked together to kill your fish.

maybe, but I'm going to concede and say Insufficient QT time, or possible unknown reason. I watch my water very close, so not ph swing, maybe ammonia spike, but the tank is 55 g, with less than 30 inches of fish (mostly tetras) and well established and planted, so I think unlikely, though it's too late to tell

Stressful medications, would've affected them in QT right? not six days after being treated? I'm open to suggestions for better less stressful meds. In the case that all these things acted to stress these fish out, wouldnt they have died but the healthy ones in tank lived? Please correct me if I'm wrong.

Im not trying to insult you, but ruling out unlikely causes i feel could find a definitive reason that can be avoided in the future.