Bad quality water will kill a stressed fish, even thou other non stressed fish seem uneffected. Conversely, good quality water will revive a stressed fish.
When returning a fish, they usually want to test your water anyway. Might as well test it now before they find out that your nitrite level is through the roof, even thou other fish are fine.
When returning a fish, they usually want to test your water anyway. Might as well test it now before they find out that your nitrite level is through the roof, even thou other fish are fine.
