Ok Sabella here is the first draft for the closed loop plumbing for your glass tank.
The concept is to have one 1 1/2 input hole for your closed loop ( I am asuming that you are going to have a pump for it that has a 1 1/2 out and input), run this pipe down to the input of th pump, Make sure you drill the hole for this to where you can easily get to for cleaning.
Coming out of the pump you will run 1 1/2 piping up the back of the tank, to just above the top edge. From thier attach a tee. Take one side of the tee and continue the 1 1/2 pipe to a point where you want to enter the tank. At this point attach a 90 elbow pointing towards the front of the tank. The attach a 1 1/2 slip by 3/4 slip adapter. Now run the 3/4 inch pipe over the edge of the tank top and then 90 down into the tank, go about 6 inches down into the water, from thier a tee, then go about 6 inches out from each side of that tee and attach another 90, but on this 90 make it slip on the side that goes onto the pipe and then threaded on the outside, make it so the 90 points to the front of the tank, do the same on the other side. The concept would be to be able to screw locline into the 90's so you can adjust them.
Do the same thing on the other side of the tank. This shold take care of the upper and middle flow for your setup. The tunzes you have will take care of the bottom flow.
If you like this idea we can move onto the balance of the plumbing
Mike
The concept is to have one 1 1/2 input hole for your closed loop ( I am asuming that you are going to have a pump for it that has a 1 1/2 out and input), run this pipe down to the input of th pump, Make sure you drill the hole for this to where you can easily get to for cleaning.
Coming out of the pump you will run 1 1/2 piping up the back of the tank, to just above the top edge. From thier attach a tee. Take one side of the tee and continue the 1 1/2 pipe to a point where you want to enter the tank. At this point attach a 90 elbow pointing towards the front of the tank. The attach a 1 1/2 slip by 3/4 slip adapter. Now run the 3/4 inch pipe over the edge of the tank top and then 90 down into the tank, go about 6 inches down into the water, from thier a tee, then go about 6 inches out from each side of that tee and attach another 90, but on this 90 make it slip on the side that goes onto the pipe and then threaded on the outside, make it so the 90 points to the front of the tank, do the same on the other side. The concept would be to be able to screw locline into the 90's so you can adjust them.
Do the same thing on the other side of the tank. This shold take care of the upper and middle flow for your setup. The tunzes you have will take care of the bottom flow.
If you like this idea we can move onto the balance of the plumbing
Mike