Originally Posted by Aoicat
I'm talking about the flexible tubing. It doesn't matter how big the vacuum part is, the water can only travel so fast through a siphon tube. That speed is determined by the diameter of the tubing.
I've found that most brands use the same size of tubing for various sized siphon ends. I typically use the largest they have for sale at the time, but I do prefer the smallest hard-plastic portion (diameter-wise) that I can find. It makes it easier to poke around some decorations without rooting up plants or making a big cloud.
If you're just trying to drain the tank but not vacuum the bottom, you can use any piece of tubing or hose, but you'll probably have to suck on the end of it to start the flow.