Hi Sam, lots of variables ....1st, we need to look at the performance chart for your pump...
http://www.aquaticeco.com/images/static ... es/MD2.gif
You said your tank is 40 inches tall, lets add a little height for resistance of water going through your pipes. Say 5 ft...according to the chart, you are getting just under 500 gallons an hour. That [i]should be [i] a good starting point. ( in theory...you could be getting 125 gph in each GB, plus the FT)

Just so we are all on the same page, here is a drawing of a chop 2 system...
http://www.ecofilms.com.au/2010/10/28/a ... -revealed/

Sam, whatever flow of water is 'going into' the fish tank, has to be coming out. If you have a ball valve at the fish tank and at the grow bed(s), you could cut back on the flow to the bed(s) and increase the flow to the fish tank.
However....This will slow down the 'fill time' for the GB's...your bell siphons may not kick in, due to lack of incomming flow (?)



Is there a problem with the fish and/or system ? or your just concerned because it doesn't look like much flow into the FT ?
Are you concerned with the DO ? Do you have a spray bar on the water going to the FT ?
You might consider a small air pump in the FT. If you get one with a built in battery backup, that will be an added safety measure.

Of course, you already ran your pipes...I try to go one size bigger on the supply line, from the pump. This gives 'less pressure/resistance' and a little more flow.
Also, keeping the amount of 90's to a minimum helps. I use 2- 45 degree fittings together, instead of one 90 degree. Yes, it's kind of overkill...but it is less resistance.

Lots to consider...