Doubt that will work very well the water level will just rise possibly causing your ac300 to over flow, to decrease the water flow you need to cut off the intake, not outlet, try this, take the tube that sucks the water in off and pack it will some foam, the bigger the pores the less reduced the smaller the pores the more reduced, if you take some filter floss and stuf 1/2 inch followed by a tight fitting circle of a bio-sponge type foam(also used for algae clean up, large pores) to block the folter floss from getting into your impeller, I think that will achive the exact amount of reduced flow you want
got a question for you. I didn't use a baffle between the intake and the main chamber. Right now I have a millenium 1000 poly/carbon pad there which fits great. I'm still getting alot of flow into the 2.5 even though I swapped out the impeller with the AC mini impeller, and I have the intake shifted a bit to the right since it seems to struggle when fully seated. Does the baffle really make much difference in reducing the flow? I was thinking that I could take a piece of acrylic with several 1/2" holes drilled across the top and place it right where the water flows back into the tank. I figured this may slow things down a bit and also help keep my chaeto from drifting down into the tank with the flow. I get strings of chaeto hangin into the main tank. not pretty.