The return pump for marine aquarium is absolutely necessary, but often you can read that many aquarists made wrong choices. 
Qui l’articolo in italiano.
We wrote about this topic many years ago, but today we can make clearer some concepts that may help you. Anyway you still can read the old article here. At that time we focused in how calculating the flow rate, now we’ll see how choosing the correct one.
Traditionally we have an aquarium connected to the technical compartment where there’s the skimmer for the water filtration. Ideally we should be able to put the skimmer directly in the tank, without the sump, and obviously without the return pump. Unluckily that’s not the case.
The role of the return pump
But thanks to this ideal solution we can understand what’s the role of the return pump. It feeds the filtration system, or the skimmer. Let’s imagine now that could be possible to have an ideal system, but with a sump. All the water coming from the aquarium should be filtered by the skimmer and than again it would come back to tank.
Moreover the return pump oxygenates the entire system, even if the major contribution is given by the skimmer.
Here’s the first big mistake of the aquarists: choose the return pump by the volume of the aquarium. This choice hasn’t a physical sense, because it doesn’t lead to a better filtering. Instead it needs more energy for bringing the water from the aquarium to the sump and again into the aquarium. The water isn’t treated anyway.
Unluckily that’s true that sumps aren’t always well built, and sometimes they haven’t a separation baffle for the skimmer and the return pump. It’s necessary to over estimate the range of the return pump from a minimum of 20% to a maximum of 50% of the real values. Lately skimmers with low drains are in fashion. But these types don’t facilitate the exploitation of the sump and electric current. Right now I have one of these skimmers, the Vertex Omega 180i. But skimmers with an high drain can take the water directly from the main compartment of the sump. So they can drain the treated water in the sump compartment of the return pump. In this way all the process is optimized and we can perfectly set the return pump.
Oversizing of the pump and filtration: some considerations
Many aquarists say that bringing more water in sump helps the aquarium system. We have already declared that’s nonsense. In fact we can assume that the skimmer works in the same condition as it in aquarium (as if it has been put in the tank). So, between the skimmer and the return pump, it is the former that determines the system operation.
Sizing of the pump and filtration: some considerations
At the same time other aquarists use a return pump with a smaller range than the ideal one. That’s absurd, because if the sump was “treated“ at a higher speed there would be a depletion of the pollutants in the sump.
The skimmer would work worse and with a bigger effort.
And with time it would stall because it would have more clean water in sump (in certain limits) and if even the little water drained in sump was perfectly treated, we would have a system sump similar to a big skimmer, limited by the range of the return pump. It’s not so easy to understand, I hope it’s enough clear. If not, feel free to ask in the comments and I’d be glad to explain better.
In a simplified way, if we had a skimmer more powerful than the pump we would end in using it under its possibilities.
The choice of a return pump
Now we have to choose our pump and we necessarily have to begin considering our skimmer. How many litres treats it? Which type of drain does it own?
We increase the range of our skimmer of 20%, so we’re sure that all the water will be treated. 10% it’s enough for a skimmer with a high drain and a partitioned sump.
The problem now is to understand which pump could have the range we need.
In the next week we’re going to analyze some pumps. We’re going also to update our excel file so you’ll have a simple and easy instrument for your researches.
So, in order to choose the perfect return pump:
- It have to be determined the real range of treated water of the skimmer;
- This value has to be increased from 10 to 50% to be sure. It depends from the construction of the sump and the drain of the skimmer;
- It has to be searched a return pump able to make circulate that water volume.
