Dear Fields of Blue,Thanks for all the responses, I'm relieve to know that my betta is comfortable and probably in safe conditions. The miracle grow was just a half empty bag in my garage which I used because I didn't want to go out and buy more soil or dig up my yard. It's of the average "Potting Soil" variety.
Also, I believe I've found out why my rasboras were having trouble breathing the other night! I noticed that my waterfall filter's output was very close to the water surface, particularly less than 1/4 inch. This allowed the water to flow down into the tank without having to break surface tension, so the amount of air being pulled down with the water was nominal at best. I've reduced the water level about an inch so the filter output now splashes down and pushes plenty of air bubbles down with it. Needless to say, I'll be closely observing my fish every night until I'm positive that the filter will provide plenty of oxygen for my fish.
I think I'd continue to follow your instincts here. If your fish are gasping for air, they need oxygen! You were right to bump up the oxygenation via your filter's splashing.
NPTs are going to need more oxygen when they are first set up. Soil bacteria are invariably consuming lots of oxygen and producing lots of CO2. So right now the plants aren't hurting for CO2, but the fish could be hurting for oxygen.
Once the substrate settles down, the situation changes.
So for the first 2-3 months you may need to change water and maintain good oxygenation to keep the fish healthy.
Your substrate (Miracle Gro Potting Soil) is a little richer than what I'd advise for a beginner, BUT it will work very well for the long term once you get through the "break-in period". Its what I've got in my 45 gal, and I love the results. Just make sure that the tank is getting enough good lighting to match the substrate richness.
Since your plants are doing well, I'd venture that you'll be fine.