You should have your filter's output set up in a way that you get surface movement. If you have that, you really shouldn't need an airstone. I read your routine on your site/blog, which eliminates some possible problems.
If some of your fish died, here are some possible problems:
1) If you make a lot of changes to your landscaping and kick up some dirt in your tank to make the water cloudy, it is always good to do a big water change at that time (50% would be good) -- do this right away.
2) Flow problems? Although the gph of your filter is probably adequate, it is not overwhelming. I'm a little underpowered on my filtering, so I keep a close eye on my flow and keep the filter (and added pre-filter) very clean. The cleanliness of my pre-filter and filter makes a huge difference in my flow. I also have to keep my plants pruned carefully to avoid blocking water flow too much.
3) Although I doubt this would kill your fish, hopefully you turn off the CO2 about an hour before you turn off your lights.
4) If you been doing anything out of the ordinary or if anything out of the ordinary happened, this could have also caused a problem. But, there is no way for any of us to know that one, unless you mention it, i.e. -- long power outage, heater left unplugged, medicines in tank, etc, etc.
Edit: Sheesh, that's embarrassing! I'm sorry, I did not look and see that this was the Talk to Tom Barr forum when I responded. Sorry Tom and Rick.