Last year I did a large water change on my tank 50% (I usually only do like a 20-30% per week) and it was at a change in the seasons (Spring) and afterwards my fish were all gasping hard. I ended up losing a few fish one of which being an 8" Gold Saum male (my fav fish). Prior to losing any stock after I noticed the condition I did another WC right away, added air stones, moved the outlets of my canisters to cause more surface agitation and just anything I could think of (including posting on MFK). The most likely cause, which I do think is the reason now looking back, is that my city was flushing their systems and had higher concentration of chemicals in the tap water.