As a general rule of thumb, you want the water in your plumbing system to never be traveling more than 5ft per second as far as efficiency is concerned. Keeping it lower than that prevents friction inside the piping and the velocity head from ever contributing significantly to added resistance on the pump, which would reduce flow (possibly significantly) while still using the same amount of electricity. A 5 ft/s velocity of water in the plumbing system is the same resistance as far as the pump is concerned as it takes to pump water vertically about 6”, so 6” of head pressure is what the pump sees. Water flowing at 10ft/s through the same system is the same resistance as water being pumped vertically a little over 1.5’, so as you can see it is not a linear relationship. The faster the water goes through the plumbing system the more head the pump sees and your waste losses compound very fast when you go over 5ft/s velocity in the pipes. This wastes pump capability and electricity and you don’t gain anything. Using larger pipe at least for your main vertical feeds for your setup (at least ¾” pipe) will be the cheapest bang for the buck that you could ever do as far as your entire filtration and circulation system is concerned. ½” PVC is about exactly 0.622” inside diameter, so to keep the water going less than 5ft/s for optimal efficiency there should be less flow than 5ft/s x 3600s/hr x 12in/ft x 3.14 x 0.311in x 0.311in.

0.311” is the radius of the pipe
3.14 is the pi multiplier for finding the area of a circle (pi x radius squared is the formula for the area of a circle)

This calculates out to roughly 65,600in3/hr. There are 231in3 in a gallon, so that is 283 gal/hr flow. That means that ideally you want no more than 283 gal/hr flow through each of your risers in your ½” system otherwise you begin losing a lot of efficiency, cutting flow capability, and adding a lot more needless resistance on your pump with no net gain at all. Going much higher than that number will severely restrict the total flow that you end up with through your system as a whole.

If you go to ¾” pipe on your verticals that feed the manifold, you just replace the 0.31” radiuses in the equation above with 0.824/2 or 0.412”. If you calculate it all out you find that anything up to 500gal/hr for each pipe is a safe maximum. You can do the same with 1” as well and you end up with 807gal/hr for each pipe.

Now in as far as making sure that you get good flow to all parts of the system are concerned I have a few recommendations. I agree with you, there is no reason to put T’s along the front of the tank, but the manifold pipe running across the front and connecting the two sides does serve a valuable purpose. It balances the water flow and pressures equally throughout the entire manifold. Think of a long 12” airstone with a single input at one end. Most of the bubbles that flow out of the airstone will flow out within the first few inches and then the flow will begin to drop off unless there is a lot of reserve air pressure in the line to push all the way to the other end. You could get the same amount of bubbles and do it more efficiently having the air enter the airstone from both ends in a loop configuration. This would create equal pressure throughout the entire airstone, and you would get a nice even concentration of bubbles across the entire 12” of stone. That difference and more evenly balanced flow is what the pipe across the front of the system that makes the loop is doing for you.

Ideally, your setup would have the best flow and least resistance (assuming that you have more than 1000gal/hr of pump flow which is possible with that size tank) by having 2 1” verticals that feed the system. Then T down to ¾” and run ¾” for the entire existing under substrate setup that you have now in your photobucket pictures. Just use T’s with ½” third legs though instead of ¾”. Then add an additional pipe across the front of the tank to connect the two sides, and run that in ½”. ¾” is not necessary for just balancing flows. Also, still make all of the vertical spray bars out of 1/2”. That will equalize all of your pressures well, and you will get nice even and equal flow out of everything with the water traveling about the same speed everywhere through your plumbing setup. Changing the diameters of the pipes in this way and making them smaller as you travel towards the ends of the system uses the plumbing itself to balance flow rates out so there is much less of an issue with trying to balance everything out by number of holes and hole diameter in the vertical spray bars. It will still be an issue, but it won’t be nearly as sensitive of an issue and problems will be minimized.

If you have less than 1000gal/hr of total pump flow, just do the bridge along the front to connect the two ends to form the manifold loop, and then feed the entire setup with two ¾” pipes instead of ½” pipe and leave the rest as you already have it and you should be fine.

Good luck, Jeremy