Using fluid aura this way is typically ok, though if the tank doesn't feel comfortable with I'd just leave them be. Sometimes tanks will panic when they see mobs ping ponging around, and simply feel more in control with everything in flash/overpower range. The mistake that most conjurers make with fluid aura is knocking back a mob that they've pulled hate on, causing the tank to scramble around and lose focus on the primary target, thus losing hate to dps, etc. however, I see no issue with using the skill as you've described, as long as the tank is cool with it.