How big is your tank? Adding a bunch of fish in one go would cause an ammonia spike. Ammonia burns their gills and they suffocate (would look like they are gasping). You have to add fish slowly, just one or two at a time depending on tank size and fish size so that the nitrifying bacteria can break down the wastes into less harmful nutrients. You don't have to add anything fancy or special to the tank, just go slower and take your time when adding fish.
Roxy - Feb 2005 - 75 lb American Staffordshire Bull Terrier (adopted Jul 2011)
Peewee - Jan 2006 - 8 lb Chihuahua (adopted May 2009)
Myka - Nov 1998 to Jan 2010 - 85 lb American Pit Bull Terrier cross
Lacy - Sept 1992 to July 2003 - 18 lb Reg Shetland Sheepdog