Pump Purchase Primer

Today, let's get into a little primer on pumps. Yes, that was an intentional, if bad, wordplay. (Some pumps must be primed... never mind). Anyway, if you are new to planning plumbing, one area that can seem overwhelming is pumps. For a number of reasons, there are external pumps and internal pumps, they all have different “heads” or “head pressures” and critically, how does one pair this pump with an aquarium and balance the fill / drain of the aquarium system?

External pumps are exactly what they say they are; they live outside the sump system, and thus stay dry. These are usually the more expensive option but are frequently the quietest and most powerful. They also have the benefit of very little heat transfer, as they use the air around them to cool the motor inside. By comparison, internal pumps live inside the final chamber of the sump and are submerged (another name for them). They use the water around them to cool their motor, and thus always transfer a little heat to the water, this can be a benefit for many hobbyists who live in cooler climates and need to heat their tanks anyway. Internal pumps are a little bit cheaper than their external counterparts, and on the top end of the scale are not as strong. Aquarists that work here have plumbed many tanks both ways and continue to do so.

It would be easy to say something like “if you have a tank that is 200 gallons, you need an external pump,” and indeed if you need a lot of flow, at this tank size, that would probably be the most efficient way to do the job, however, in a lower flow set up, a large submerged pump would likely work, and in some instances, you could even use two! So how to decide what to do?

Let's look at your tank and ask some relevant questions. First, how big is the tank? For an example, we'll use a very popular aquarium size in north America. The nearly ubiquitous 55-gallon tank. Conceptually, we also need to talk about what your plan is for stocking the tank. We're not going to get into every fish and invert you could put in there, but what kind of overturn are you going to need? A full blown SPS Reef tank might need overturn of 20 times per hour, plus additional flow devices. A fish only tank with some live rock, might only need an overturn of five or ten times per hour. You can see now why these ideas are related. If you have a 55-gallon tank, and you need to filter that water 20 times per hour, you need a pump that moves 1100gph, a number that will likely be impossible without multiple overflow drains. For the tank that only needs to overturn the water five times per hour, that pump would have to move 275 gallons per hour.

These are not the only two variables, and this next one is very important. How is this tank drained, and what is maximum the flow of that system? Whether your tank has an overflow built in, or you are using a hang-on-back overflow from any of the myriad of vendors who supply them, we need to find out how fast that system removes water from the tank, otherwise, you might pump water back into the tank faster than it can leave, with disastrous results. Capacity for flow out of tank must be higher than the volume of water going back into it, or you'll dry out the sump and fry your pump (and perhaps overflow the tank, which can really bring out the worst in roommates.)

So, if you have an overflow that is rated for 600 gallons an hour, then this is the maximum gallons per hour that your pump can supply without causing an issue. We generally say that because manufacturers estimates might be off a little bit at your altitude, you should leave at least a 10% margin for error in this calculation. So, if your aquarium can drain 600 gallons per hour, you should really max your pump out at 540 gph. You don't need to worry about underpowering your pump, except for as it applies to the filtration capacity of the tank.

On the topic of underpowering, we need to talk about something called head. You see, most pumps are marketed with the maximum gallons per hour that they can achieve. This number suggests that the pump is moving water out of, and into containers at the same level it is on. As you start to pump water higher, the gallons per hour will decrease. For very small pumps, this might go so far as the render them ineffective. So, while planning your pump, think about how high the return is above the pump. If the return comes in over the top of the tank, and that spot is four feet higher 

than where the pump lives, you'll need to make an adjustment. Luckily, pretty much all pump manufacturers supply you with a chart to show you how the flow decreases with height. In our 55-gallon aquarium with a 600-gallon overflow, you might need a pump that is actually rated higher than 600 gallons, because it needs the extra power to actually deliver the 540 gallons you are looking for, at four feet of head, and in this case, a pump like the Reef Pump Compact 2500 from TMC would be a great choice. https://www.qualitymarine.com/quality-marine/supplies/pumps/t-m-c/reef-pump-compact-2500-dc-aquarium-pump-0740034/

So, what happens if you chose a pump that can't move enough water to that head? It won't work, and you should either return it, or find another use for it. Maybe you have another, smaller aquarium or one with flow needs that are less intense? That's a perfect home for it. Maybe it's going to be an awesome salt mixing pump?

What happens when you bought a pump that is too big? Well, you might also be able to return this, but many pumps have flow that can be adjusted with a valve, and when doing plumbing, having a valve on both sides of your pump will allow you to remove, clean and if need be, replace that pump without making a big wet mess. If you want to adjust the flow, only use the valve on the output. Never pinch the flow of a pump on the intake side, this will cause a pump to try and suck more water than is available and causing it to cavitate. This will make a horrendous noise, create bubbles and in not that much time, ruin the pump.

Lastly, and back to the priming pun, never run any pump dry. Make sure that there is water in the wet side of a dry pump, and that submersible pumps are submerged before ever plugging them in or turning them on. We here at Quality Marine supply a very nice, quiet and powerful line of submersible pumps and other flow devices to our vendors for you to use in your displays from TMC as well as a full line of professional grade external pumps. As with all thing's aquarium related, there is always more studying to be done, but we hope this will get you started down the right path of picking a good pump for the display you're planning! Head to your LFS, ask questions, and tell them QM sent you!