WHAT HAPPENED TO MY PERUVIAN LORICARIA?!?!

[Loganfish]

Yesterday I did a water change on my 20 gallon and added more sand from the 55 gallon. I put him in a white bowl to hold him while I do maintenance since he didn’t fit in a cup with the other fish. I put him back in and he seemed fine. Hours later I come up to see him half dead. This morning he is presumed dead. What happened? I used declorinator, plugged a heater back in for him, etc. No signs of parasites or infections, as well as injuries. So what could have happened? It took me so long to get one of these. He was eating like a hog and was thriving. I am so pissed. First pick is him today presumed dead, the other pick is him yesterday looking healthy as can be.

 

[SilverArowanaBoi]

Yesterday I did a water change on my 20 gallon and added more sand from the 55 gallon. I put him in a white bowl to hold him while I do maintenance since he didn’t fit in a cup with the other fish. I put him back in and he seemed fine. Hours later I come up to see him half dead. This morning he is presumed dead. What happened? I used declorinator, plugged a heater back in for him, etc. No signs of parasites or infections, as well as injuries. So what could have happened? It took me so long to get one of these. He was eating like a hog and was thriving. I am so pissed. First pick is him today presumed dead, the other pick is him yesterday looking healthy as can be.

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I had similar happen to my Royal Loricarrid. Sorry about that.

 

[Fallen_Leaves16]

Could be a lot of things, really. No sure-fire way of knowing for certain, though I'm almost positive it's some sort of husbandry error.
I do find it rather odd how you thought it was necessary to transfer the inhabitants into a cup, of all things, during maintenance; or even remove them during a water change at all- were you completely draining the tank and resetting it?
They are rather sensitive fishes, but not to the point of immediately dropping dead after an event such as a water change- something must've drastically altered the parameters to some degree.
Does your 55G have the same parameters as the 20?
The fish does seem rather distressed in the second image, certainly not "healthy as can be"- slight discolouration, head raised above substrate, slight indentations along the skull, somewhat frayed scutes, etc. Seems oxygen-starved and stressed, possibly due to suboptimal water parameters.

 

[duanes]

The Sturisoma (whiptails) I catch in Panama come from highly oxygenated parts of rivers, and if kept in non-moving water for extended periods without aeration seem to weaken quickly.


So when I catch them, they are put in a cooler with battery operated air pump immediatly, for a couple hour drive

And that waters they come from (beside being flowinng and highly oxygenated), are very low in nitrates, so they get very fequent water changes in their tanks (30% every other day) to simulate their natural environment.
The right tube is a nitrate concentration in rivers they come from, left is pH

If left in a non-aerated bowl for even an hour or 2, without frequent (hourly) water changes may have been quite stressful, especially if the ntrate concentration was over 5 ppm