After reading the thread on increasing flow in 90g tank, thought that I also may get help with the same problem .
I have 90g tank too, undrilled, overflow box is at the right side wall, near the front. Behind it is Koralia 2 (600 gph) powerhead, directed to the back and top, making some ripples on the surface for oxygenation.
Left top corner, on the back wall, Seio 620 (600 gph), directed at the same level to the front wall, tot at the middle, maybe one third of the tank length.
Both powerheads are creating flow around the tank, more at the top, than at the bottom, with flow pushing water right to the overflow box intake (most articles say it should be done so).
Return pump's flow (~500 gph) enters tank at the right top, some of it blows at the top, where the anti-siphon hole is, and all other goes to the spraybar along the bottom, behind the rock.
There are no space between rockwork to place additional powerheads on the back wall, blowing onto front wall, with dispersed flow washing the front of the rocks with corals. Actually, there is space (medium rock pieces are laid as a brick, with a spaces in-between), but powerheads couldn't be removed for a cleaning. And I cannot change aquascaping too much because of the shape of LR pieces, that I have: this is the only stable configuration, and it took hours to find it by trial and error.
What else I could think about:
I placed the second Seio 620 on the left side wall, close to the front, directed down under angle onto the front glass near it: very short pass, then it is reflected onto rockwork, mixed with flow from the first Seio. This way I have back to front and again back flow.
If you still here and continue reading (good!), the rockwork is L-shaped, covering left side wall and 2/3 of adjacent back wall. Right one third of the tank is empty at all so far, reserved solely for tube anemone, 1 ft in diameter, that occupies all this place. The flow should be relatively slow here, for tube anemone. With no powerheads below the top - otherwise the tentacles could be caught.
I would like to increase flow in the middle of the rock and in the left back corner, filled by rock. Maybe even making two sets of powerheads on timer for creation wavemaking effect (every couple of hours, maybe - my timers allow 6 settings a day).
I also have Sequence Dart (2400 gph) pump, that I thought to use for a short time mixing action once in a few hours, but it was before tube anemone, and was advised against the DIY unskilled installation.
It's all. Could I improve this somehow, without drilling the tank and redoing rockwork?
So far received in other places advices were:
1. It's up to you. Nobody will tell you, how to place powerheads in particular rock arrangement.
2. Hire the professionals from LFS.
Do I have another, DIY options?