At least 3 times out of 5, gasping fish can be attributed to ammonia or nitrite poisoning. These chemicals quite literally burn their gills, making it hard for them to breathe...
Oxy-dep is another thought, but it's not nearly as common...
What kind of fish do you have? There's only so many species that care much for a ph over 8... this could also be contributing to your problems, especially if you have ammonia present. (Temp as well could be an issue, but it's not likely...)
This has been going on in my quarantine tank. It's a 5 gal bow with a penguin mini biowheel well cycled. I had 2 danios, 2 small otos, 2 small angels, 2 guppies and 2 neons. I was doing 30% water changes every 2 or 3 days using Tetra's AquaSafe. I lost the 2 guppies, the 2 angels and 1 neon.
Can some levels of ammo/nitrite/ph change during the night? My ph is kinda high at 8.2 (gh8kh5) but have never measured any detectable ammo or nitrite.
They were all new fish except the fancy guppies, amazingly the otos are doing great, I normally lose some.
thanks again
I went and checked my Fishlopaedia and it said that "fish gasping at surface" was a sign of hypoxia, or oxygen starvation. The causes are a mixed bag of whats already been mentioned. Lack of O2 or excessive CO2 can cause it. Ammonia or nitrite poisoning, by damaging the gills, also causes it (Ammonia is also more toxic at higher pHs, so it wouldn't take much). Alkalosis (fish being exposed to higher pH levels then they're prepared to deal with) can also cause inflammation of the gills and hypoxia.
One way or another I'd think its probably related to the high pH, although I'm surprised it got the guppies and danios and not the ottos. Is your lfs in a different water system or treating their water? It may be an acclimation issue. (An employee in a big store near me told me they doctor most of their natively hard water to pH 7 because of the large adjacent softwater market).I'd try taking down the pH in the Q tank and acclimating the newcomers slowly until they're ready for the big tank.