That's pretty odd. Most of my livestock goes through a very quick, or often non-existent aclimatization.
I know what my tank parameters are, so I usually test the water that the livestock comes in. IF the PH and salinity are close, then it's just temperature shock you really need to watch for.
Around here, the LFS
are pretty good about salinity and PH. So, the specific gravity isn't usually too far off from what I have, so the aclimatization isn't too long.
I would check your own water parameters to be sure, then check that from the LFS
. If they are too different then you've found your cause.