When replacing a fountain pump or choosing a new one, first there are
some keys terms to keep in mind:

Head : This is the maximum vertical
lift of the pump. For example, a 6' head means the pump is rated to pump
water up to 6 feet high. Note, however, that at 6 feet the pump would be providing very little water, a gallons
per hour around zero. So if you
need to pump, say, 200 gph at 72”,
you will probably need  gallon per
hour pump to do the job. See a "pump curve" for the amount of water pumped
to various heights.

GPH : Gallons per hour, usually rated at different heights

GPM : Gallons per minute, usually rated at different heights

Pump Curve : The amount of water volume "curved" according to various heights. A 500 gallon per hour pump, for instance, might pump 500 gallons per hour at 0” lift, 350 gallons per hour at 24” of lift, and so forth.

Water Volume : This is controlled by a few factors. One factor is the size of the pump, as covered above. How wide should your tubing be? Tubing is measured in two ways: inside diameter (i.d.) and outside diameter (o.d.). Very thin i.d. tubing will greatly reduce water flow. Many customers are shocked when they find that, after hooking up their 500 gallon per hour pump to ½” inside diameter tubing, they are only getting what they consider a trickle. Well, about the most they will get is 200 gallons per hour or so. They have restricted the flow too much. When purchasing a pump, find out what size of tubing is supposed to go with it. Another problem is running the tubing too far. Long lengths of tubing create resistance. If your pump calls for ½” i.d. tubing, for instance, but you are running the tubing twenty feet from the pump, it is a good idea to use ¾” tubing instead so as not to cut down too much on flow.