The tube that I was useing that day was off a gravel vac. Since I have sand, the gravel vack doesn't quite provide enough suction to pick the junk off the bottom. I get such strong suction cause the bucket sits on the floor 2.5 feet below the tank. The farther down the water travels, the stronger the the syphon is so maybe you just don't have the dirty end (as I call it) of the tube low enough.