Hi, stucco, thanks for your ideas. I finally got it working but would like your confirmation that this is normal. Here are some facts:

110g fish stock tank
50g rubbermaid stock tank grow bed
Siphon tube length: 7 inches from the bottom of the bed
Grow bed depth: 9 inches

It takes approximately 20 minutes to fill the tank now to siphon starts and it takes about 6 minutes til the siphon breaks. Are these figures normal? The flow rate coming in looks quite slow but I guess this is what it takes to work.

BTW, I use this plan for the bell siphon and I think it is one of the most intuitive on the net:

http://www.ctahr.hawaii.edu/oc/freepubs/pdf/BIO-10.pdf