I'm very impressed that your LFS takes the trouble to separate their guppies. Do they separate females? And more importantly, are they able to tell them apart?

When I am king of the world and establish "Ye Royal Fish Shoppe", I will probably display the male guppies in a large tank. I would have the females separated into smaller tanks. Digital photos of the matching males would be posted on each tank.

Ah well, it's fun to dream.

Your filter, if clean and running unobstructed, is probably enough. In larger tanks, some aquarists will run a sponge or box filter in addition to their major (power, cannister, gerbil) filter so that they have a fail safe if the big one fails. A lotta good that does during a power outage. ;)

The actual aeration, or gas exchange, takes place when the water is brought to the surface and surface tension is broken. I was surprised to discover that bubbles from the airstone actually contribute very little O2 to the water. What they do, quite well, is circulate the water to the surface where the oxygen exchange takes place.

People using CO2 units, to give their plants a boost, will often set their CO2 units to release that gas in a powerhead stream aimed so it will not shoot up to the tank surface. In time some of the extra CO2 will still be released into the atmosphere. They want to keep a set amount in the tank.

That circulation of water is also important if you have a heater in the tank. Don't use a heater w/o water circulation.

They sell powerheads with venturis so that process can be accelerated. I found their noise annoying (despite actually finding some peace and comfort in the putt-putt and gurgles of most systems)and don't even know where I stashed the unused venturi attachments. :(