There are generally a couple of ways to use tubes - and they are quite different, so it's easy to lose track of who uses which method.
One method lays the tubes horizontally and agitates by turning the tubes - either by hand in a water bath (BTZS) or on some type of motor base. With this method a relatively small amount of developer is used.
The other general method (and there are variations) is what Francesco uses - the tubes are left vertical and they are filled with enough developer to completely cover the negative. With this method agitation is much less frequent, the developer is usually more dilute, and development times are usually longer.
(Then there's my method, which is the one, true, correct method of film development with tubes.)
It's a matter of what you want to achieve, and experimentation.