14.6. Pumps – Aquarium Science

Pumps are essential to an aquarium. There are many types of pumps available:

The pumps built into most canisters are cheap external magnetic drive pumps that don’t work very well against any degree of back pressure or head. They work reasonably well in a canister because the water return from the aquarium to the pump is a siphon, this pushes water into the pump and prevents a high head on a cheap pump.
For sumps cheap submerged pumps should be avoided. They typically lack the power to drive significant amounts of water up five feet or so to the top of the aquarium.
DC pumps are great largely because they are very quiet. But they can be a problem if one lives in a rural area that often has power surges in the electric power supply. These surges can burn out the rectifiers in a DC motor. DC motors are more energy efficient and have variable speed but they are expensive.
External AC pumps are great for large sumps but expensive.
Note that cheap pumps have flat impeller blades that work in both a clockwise and a counterclockwise direction. In this way they avoid expensive starters which make the pump go in only one direction. Reportedly this can result in a cheap pump “freezing” if there is a power outage. I have a lot of power outages and have never had this happen.. soooo…
There is also the constant dialogue over magnetic drive versus direct drive. Each has its pros and cons. Direct drives are probably the best with sandy substrates as fine particles of sand can take out a magnetic drive impeller assembly in only months. Note probably 95% of the pumps used in the aquarium hobby are magnetic drive pumps.

Sump Pumps

When selecting a pump for a sump filter it is important to look at the pumps data provided by the supplier or the manufacturer. Most suppliers rate their pumps by two factors, flow and Max head. To find the best pump for a sump application it is important to know the height differential between the top of the aquarium water and the top of the sump water.

In order to calculate head pressure, use the following standard formulas to add up the vertical rise, distance, and 90° turns to come up with a head pressure number.

Every 1 ft of vertical rise = 1 ft of head pressure
Every 90° elbow fitting = 1 ft of head pressure
Every 45° elbow fitting = 0.5 ft of head pressure
Every 10ft of flat horizontal distance = 1 ft of head pressure

To illustrate the necessary machinations let’s assume one has a sump where the aquarium level is 4 feet (30,5 cm) above the sump water level. Let’s look at some submersible pump data from Drs Foster and Smith (now out of business).

In addition to the four-foott (30,5 cm) data point say we have a 100-gallon (379 liter) aquarium we want to turn-over 5 times an hour, or 500 GPH. If we go with two small pumps, like two Quiet One 1200s, one might think that 500 GPH will be achieved. But look at the “Max head” point. The 6.10 feet Max Head is where flow stops. So at four feet of head each 1200 will only be pumping about 1/3 its full capacity. One 1200 will probably only give a measly 100 GPH. So with two 1200’s we might get 200 GPH if we are lucky.

Let’s look at the Mag Drive MD9.5. It has a 950 Flow gph and a Max Head of 14 feet. Subtract the 4 foot from the 14 foot to give 10 foot. Then divide 10 by 14, or 10/14 = 0.714. Multiply 950 by 0.714 and you have 678 GPH. This is roughly what this pump will give you if you put it in your sump. More than enough to do the job.

Let’s look at the Quiet One 4000. It has a 991 Flow gph and a Max Head of 10.17 feet. Subtract the 4 foot from the 10.17 foot to give 6.17 foot. Then divide 6.17 by 10.17 to give 6.17/10.17 = 0.607. Multiply 991 by 0.607 and you have 601 GPH. This is roughly what this pump will give you if you put it in your sump. Again, more than enough to do the job.

The Mag Drive is $115 and the Quiet One is $65. Which one you chose is up to you and there are other pump lines available which will do the job.

Now for the point often overlooked with pumps. A submerged pump such as the Mag Drive 9.5 runs with a certain wattage, in this case 93 watts. Because this pump is submerged that means that 93 watts of energy is going into the water as heat 24 hours a day. This is the equivalent of an aquarium heater of 93 watts on 24/7.

The degree to which this becomes a problem depends on the temperature in your home. Very roughly 1 watt of pump can raise 1 gallon (3,785 liter) of water 10°F (5,5°C). So if you have a 100 gallon (379 liter) aquarium on a 100 watt pump to give a 6X turn-over rate. And you have a home which floats up to 85°F (29,5°C) in the summer, your aquarium temperature can hit 95°F (35°C). Add a 36 watt UV unit and you will be in trouble.

If you want 6X turnover this makes the Quiet One 1200 a better choice as it has only 65 watts of power usage. Even a better alternative is an external pump (heat is released to the air) but they are expensive.

Note that flow rate and turn-over is vastly overrated as criteria for a filtration system. Far more important is the effective surface area of the filtration media. With sufficient surface area to the filter media a turn-over rate of 2X or even 1X can be made to work.