Thanks guys. I'm basing it on the amount of milbemycin in each pill and since the med dog pills have exactly half of the agent (5.75 mg) compared to the large dogs of 11.5 mg, I'm doing 2 spoonfuls per 35 gals. Ideally, this would put the same amount of milbemycin in the water. The threads I've read also say to try and estimate exactly the volume of water you'll be treating but of couse my 125 is full of LR and corals and my sump has a turbo floater skimmer in it, so I'm just treating it for the total 155 gal and not subtracting out rock space. Is this what you have done? I figure I'll be over treating by the voulume of rock and skimmer but I should be OK. Thanks again for your help.