Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
You probably should look into it.
Is the local water (whatever system the fish store is on) relatively hard? That is, at least 2-3 German degrees of hardness harder than the water in your tank?
Fish that are acclimated to somewhat harder water have a very hard time adapting to softer water. You can research osmotic balance and similar subjects.
Fish need to get rid of excess water that enters their body.
Their metabolism is geared to handle just a certain amount of excess water. When more water starts entering their body they need to adjust their metabolism. This takes time (up to a month).
When they are moved into water that is much softer than what they are acclimated to so much more water enters their cells that they can die.
Easiest way to tell:
Ask at the store if they treat their water at all, especially if they blend it with RO or distilled. Ask what the GH, KH and TDS of their water is.
Tell them why: You want to get more fish, but want to set up a quarantine tank that matches the store water.