my 90g is 4feet by 18" wide. I have the tank essentially divided into two narrow halves with a long island down the middle that goes 3/4 to the rim. This pretty much makes each powerhead work independently on each long side. Powerheads are about midway up the tank, angled to the side glass gently.
I just got the 6205 cover for only one of my two 6105's, to try it out as you are. They are both on the right short end of the tank pointing along the long viewing glass. After reading Roger's post above I switched so both are NOW on 12v...I think it's a bit better for control. I was using 18v on the 6205 covered powerhead, but was pulling a bit of sand up (not bad) on long pulse mode going up to 60%. Certainly not a sandstorm (this was right below the powerhead), but gentle mixing of the surface. It's also worthwhile noting the sand mixing was from backtow into the powerhead, not from the outflow. I feared it would eventually create a bit of a pile. With it now using the 12v chip, I can put it up to 90-100% without the same sand mixing, plus the lower end is "lower".
I just got the 6205 cover for only one of my two 6105's, to try it out as you are. They are both on the right short end of the tank pointing along the long viewing glass. After reading Roger's post above I switched so both are NOW on 12v...I think it's a bit better for control. I was using 18v on the 6205 covered powerhead, but was pulling a bit of sand up (not bad) on long pulse mode going up to 60%. Certainly not a sandstorm (this was right below the powerhead), but gentle mixing of the surface. It's also worthwhile noting the sand mixing was from backtow into the powerhead, not from the outflow. I feared it would eventually create a bit of a pile. With it now using the 12v chip, I can put it up to 90-100% without the same sand mixing, plus the lower end is "lower".