Very nice design. It is not clear to me where the water exits your pump but if you have some place to attach some kind of a tube, there is a very easy way to prevent water from draining from the tank without using a check valve.
I made this for my Hobie tank but the idea could be adapted to any other design that has a scupper intake pump with a threaded water outlet. All you need to do is install a 90 degree fitting with a short pipe extension that can be rotated. Here are the parts I used.
When you want to make sure that no water drains out, rotate the attachment so the pipe extends above the water line. If you want to let the tank drain, rotate the device so it is rests on the bottom of the tank.
I have also found that using this setup, I can easily clean the intake when it gets plugged with kelp. I carry a 12" piece of sprinkler riser with me. When the pump gets plugged, I insert the riser in the tube and blow some air through the system. It works every time.
Bob