For something to float, all it needs to do is weigh less than the amount of water it displaces.

Figure 8.35 pounds or so per gallon. How much does a tube weigh? I'll bet it weighs less than the same volume of water.

I think a tube from a standard 15" monitor would probably take up the space of about 3 gallons. I may be way off, but if that is the case then if the tube weighs less than 25 pounds it will float.