Silicone tubing that fits the pipes won't work off the pipe if the pipe is long enough.

Silicone tubing that suits the job will not only hold onto the pipe until hell freezes over, but won't fold on itself or kink shut.

Pick the correct flex tubing for the job. It's a firm fit over the aluminum, brass, or plastic pipe and the pipe is long enough for the job.

There must be a billion sizes of silicone tubing sold. It's dead simple to find stuff that fits the tubing that's used in the vast majority of the tanks sold today. The only thing left to do then, is to insure there is enough of that tubing sticking out inside the tank for the silicone tubing to hold onto.

When you assemble the tank, try this. After you've pushed the silicone tubing over the pipe as far as it will go, give the silicone a tug. If it even comes close to slipping off, you've got the wrong size silicone or not enough overlap.