Regarding some thoughts that WD had on filter and pump design parameters.
1) Are the two separate things of horizontal/vertical tube angle and tube length pretty much separate from each other? I think I remember some discussions the gist of which was that tube length, where inner surface area of the plastic itself, is one of the main things determining "work" for the pump is a major factor. In other words the longer the tube the stronger the pump head needed to acheive the same flow rate, or something like that. That's true, right? Basically I remember being surprised that the resistance was a much as it was.
The height of the water surface above the filter impeller is needed to provide what is called net positive suction head, NPSH. NPSH is the difference between the pressure at the suction point of a pump, like the impeller in your filter, and the pressure where that fluid would boil fluid, for us its water, at the existing temperature of the fluid. We are working with water at less than about 85F, 30C. That pressure value for water is 0.5958 psia. Since the pressure of the air around us is about 14.7 psia, the difference available to keep the water from turning to water vapor at the surface of the water is about (14.7-0.6) or 14.1 pounds per square inch. Each vertical foot of elevation provides about 0.43 psi additional pressure so the requirement that their spec implies is an NPSH of (0.43 x 2) + 14.1 which comes to 14.96. My best guess is that their engineers have specified a NPSH of 15 pound per square inch and they have done the math for you.
Horizontal displacement has no impact at all on the NPSH available but hose length does. Hose length drops the available NPSH by having the flow through that tube length gradually erode the available pressure due to line losses. The filter medium chosen and the amount of dirt it has accumulated also affect the NSPH available. My own guess is that they might have allowed 2 or 3 pounds per square inch of losses when calculating that NPSH so lets say it is really a need for 12 psi of NPSH.
The pump's impeller design, along with the shape of the inlet and outlet cavities where the impeller is located, along with the rotational speed affect how much NPSH required a pump has. For the typical simple impeller designs that we see in a hobby type aquarium pump, the real impacts to a design are the length of the vanes and the rotation speed. Curvature of the pump impeller vanes and similar parameters are seldom anything but the simplest of straight vanes in our impellers.
2) The thing of having the tubes be more horizontal or having more of a horizontal component to where they're going I would think would only be a factor to the extent to which it contributed to overall pure vertical height or added to tube length/resistance, but I guess I could be wrong. Its sort of like one of those things where if you hadn't sat in civil engineering classes you would never know what some important factor was, like the sized of boulders surrounding the footing of a bridge piling or something like that, lol.
Agreed, the main effect of the tube angle is that a more horizontal angle means longer tube runs and a larger line loss component to the NPSH available calculation. Here the manufacturers never utter a word about horizontal displacement because they have given you a hose that is only so long and probably used the entire hose length in their assumptions.
3) (ok, I know I only said a couple but.. why not? laugh.gif ) Cavitation... so rabbut's got me interested up there in something that I'm sure is a known thing (at least for ship designers, lol) and that is whether cavitation will occur regardless of whether the surface can be "tapped" for air? In other words, if you spun a propeller fast enough on a Jaques Cousteau type deep sea diver vehicle, would the propeller separate air from the water and "cavitate" itself or is that only something that happens when surface air is yanked down into the propellers path??? The point of the question being of course to understand the conditions under which our pumphead impellers could get themselves into a cavitation situation (?)
Cavitation comes from multiple factors but the main one is that the pressure drop at the impeller inlet can cause the water to momentarily flash to cold steam, the kind that can and does exist at 85F, 30C, due to the very low pressure. As the water/steam mix goes through the impeller it changes back to water as the pressure rises. If you have managed to cause cavitation at the impeller eye, the place where the water is first accelerated at the pump suction, your impeller will make a noise like a box full of marbles rattling around. Don't forget hat the volume occupied by the steam at 85F is 33765 times as great as the volume occupied by the same mass of water. The rapid change in volume is heard by us as that rattling sound of a cavitating pump. The small bubbles that can come out of solution at the reduced pressure is much less than the volume change caused by rapid boiling and condensing of the working fluid, water.
Summary, for those bored by what I said above.
Make sure that you don't try to skimp too much on the stated impeller requirements and if you do, clean the filter and inlet tubes often to keep the pressure drop through those components to a minimum. Air bubbles will have a similar but less severe sound to cavitation but will not destroy the impeller nearly as fast as the pressure spikes at the suction caused by cavitation. Cavitation at the pump suction, because of inadequate NPSH, can destroy an impeller completely in under a day so don't muck about with that requirement.
Winterlily is on the right track with minimal hose length and adequate vertical height of the water surface above the impeller.
Basically, the minimum distance is there to avoid air leaking into the filter when it runs, as the negative pressure from the motor can lead to air-leaks unless sufficient pressure from water weight in the hoses can be applied to the motor head. Even if you didn't have the 24" of distance though, you'd be unlikely to ave an issue, unless your pipes go strait down into the filter, rather than across from the tank like most people's set-up's do 
You made that rather long-winded Waterdrop 
