i am currently running a cpr overflow on my 55. it works. would i buy another cpr overflow? no. why? because it's a pain in the ass and i'll sleep better after i've moved everything over to my 75 that has an internal overflow. cpr overflows require a suction device to remove air pockets/bubbles that accumulate inside of the overflow where the water flows up, over, then back down to the back where it drains to your sump. if these air pockets are not removed, you WILL loose your siphon. i used a powerhead for a decent amount of time to remove these air pockets, and you could see the powerhead shoot microbubbles into my tank very often. currently i'm using an aqualifter pump and i like it better than hooking the overflow up to a powerhead because the aqualifter is an external pump, looks sorta like an airstone pump with 2 hoses. cpr overflow's have a
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on the top of them, so you hook the intake up to it and the return line into the back of the overflow. if you buy an external overflow, get a lifereef.