Dead spots in DT

Hey guys...so I have a 90 gallon...it is 36*24*24...so its got some height and depth to it, but not long. So The tank is over a year old...phosphates are at zero...per a salferit test kit, and nitrates stay under 5PPM...its a reef tank and I get some GHA that grows in the front left and right sections of the LR? You can all imagine my LR does not go super high...it probably goes up a foot and that's it...and my POWERHEAD are each on the top left and right of the tank and kinda point down ward in a crisscross pattern. Which I think works pretty well...I run an 18 Watt UV sterilizer made by emperor aquatics, so top of the line and DO have it set up properly, and it stays on I guess 8 hours a day...maybe longer. Do you guys think I just need to add two little baby powerheads around those rocks like something that puts out 200-300GPH? Or what do you think that's from...I feel like its dead spots but idk...