On the 75 gal I had 2 Tunze streams giving me most of my flow. With two powerheads, you usually get great randon flow by putting one powerhead on either end of the tank and have them pointed at the same spot in the middle of the front glass where it collides. Really mixes up the flow alot. One pump was rated at 1600 gph and the other 1850 gph on a wave maker/controller. Then if you look at the top, that nozzle is from my sea-swirl which was attached to my return pump from the sump. That rotates back and forth continuously. That was on about a 700 gph pump. Here is what the tank looked like without the closed loop. This is how it stayed before I took it down because I really didn't like the over the top closed loop once I put it on as it made the tank look a bit too cluttered due to my aquascaping layout etc.
So I wanted a bit more flow and didn't want anymore powerheads in addition, I didn't want to drill the tank so I did a closed loop over the top.
first step is the suction
I didn't want to see the white pvc pip in the tank and the black strainer so I made a cover out of blue acrylic like my backwall was to hide it and placed the suction for the closed loop in there.
red arrow - cap you remove to prime the closed loop first time you use it. Just remove the cap and pour water in it until the plumbing is filled with water then replace the cap. Doesn't have to be glued.
yellow arrow - suction for closed loop in the tank
aqua arrow - my diy cover LOL
This shot shows the pump I used for the closed loop sitting under my stand over my sump on a shelf
Then all you have to do is make up your own return line to the tank. I made a manifold out of pvc I painted black and used some lockline to direct the flow
Sitting on the tank
It was a bit too much going on for me so I eventually removed the closed loop, but you can see the two nozzles coming down in the front of the tank flared out. There are tons of way you can do an over the top closed loop, but this is just an example. My tank didn't have a lot of rockwork to hide the plumbing and this didn't have any corals either so everything stood out more. on the 38 gal, you couldn't see much of the plumbing.