Decaying leaves give off ammonia (NH3/NH4+) and will consume O2 as the aerobic bacteria do their work, but it would have to be a lot of decaying leaves to become overwhelming. Understand that ammonia levels of .1-1 ppm are optimal for fish, and that their rate of growth decreases with increasing NH3 even within this small range. This is where your biofilter does it's work. Not going to lie...that's a pretty random question to ask. Are you comfortable with having leaves all over your fish tank and on top of your grow beds come Fall? Are you willing to keep up with the maintenance involved? Do you have appropriate guards around your pump to account for build up of detritus that would otherwise clog your pump?
There are more concerning variables in regards to your system, I'm sure. You should really maximize the amount of sun that your plants get. Building your fish tank into the ground would help with insulation and maintaining more stable temperatures in your system, but obviously you wouldn't want to do that directly under an oak tree.