I have a Criticool Waterplant (kinda what your res looks like) and a DD12V-D4 pump also. That little pump pushes some water and causes a lot of turbulence in mine also. My mix is VW/Audi blue antifreeze (10%) and distilled water (90%). Anyhow, I've got the reservoir verticle and the hose coming out the bottom, but also have a ton of bubbles. I started tilting the computer back and forth and running the pump from another computer's PSU also trying to clear as much of the bubbles out that I can. Hopefully I'll be able to get that to work and stay bubble-free. I may have to pull the input barb and try to superglue (or something) an extension of hose on it to raise the opening of the input barb up a few inches to try to cut some of the turbulence down.
I'd rather not drop voltage to the pump to slow it down at all, but we'll see.