this is a self adjusting pond overflow or surface skimmer that i made for my koi pond. you also need to get 4" PVC pipe as a shaft so the overflow only move up and down printed using 100% infill because i you want it to last. ABS or PLA is fine, scale it up 5 times or until the diameter is 15 cm. after printing fill the outer cavity with some foam, i use polyfoam strips but any foam will do fill and keep testing to match with the system your using to suck the water surface. i'm using some of my pond gravity filtration system to do the suction so i only need to set the top surface about 1/4 of an inch above the water line.