Myth: Pump power consumption has a significant impact on temps.

Reality: It is difficult to know exactly how much heat a pump dumps into water, but a good rule of thumb is the following:

Most inline pumps dissipate 70-90% of their heat into the water, while a submerged pump dissipates 100% of its heat into the water.

A dual 120 mm radiator is good for about 0.03 C/W with a decent pair of 120 mm fans. That is, the water temps will rise 1ºC for every 33 watts in the water. So, if your pump dumps 33 watts into the water, water temps will rise 1ºC.

Therefore, the difference between a Mag3 at 40 watts and an Iwaki WMD-30 at 90 watts is fairly insignificant; 32 watts into the water versus 72 watts, so about 1.1ºC. Note that the performance of a waterblock will improve with diminishing returns as the pressure increases.

Myth: A pump's flow rate is the only consideration to make when choosing a pump.

Reality: A pump's maximum head pressure is just as, if not more, important. Many waterblocks are relatively restrictive, and most aquarium pumps are not made for high pressure drops. In order to estimate one's flow rate, calculate all pressure drops, then overlay the result on top of the pump's P/Q curve. In other words, it's not easy, but consider head pressure, too.

Myth: A T-line must be at the top of the system or water will leak out when you take the top off.

Reality: If the water were to leak out, air would have to replace the water. Since the rest of the system is sealed, air can't get in to replace the water. Thus, no water will leak out of an opened T-line unless there is a leak in the top of your loop.