I've recently upgraded from T5 to led and purchased the hydra 26 HD with the flex arm mount. Upon initial installation of the mount on my 29 Biocube, I had some issues keeping the mount securely fastened to the top of the tank. So I wanted to post something to help anyone out who may come across the same issue.
The clamping bracket that comes with the hydra 26 Flex Mount, although it says it's for rimmed aquariums, is across poor fit. The lip of the rim creates a void between the clamp and the tank, allowing the bracket to rock back and forth and subsequently working it's way up and off the rim of the tank.
To fill this void I used a 1/4" rubber mat that I cut into strips. On the inside part of the clamp face, I fastened (crazy glued) it onto the bracket so that it fits just under the rim. On the outer clamp face I had to glue 3 and a half pieces together to fill approximately a 7/8" void. These pieces were wedged between the tank and the clamp face on the outside back of the tank. The mount screws were able to be tightened down without the mount rocking and slipping off the rim.
Just an FYI for anyone that maybe be contemplating this light/mount. Although it took a little work to get a proper fit, I'm very pleased with the mount. Any questions or if I can explain something further PM me.