What is a return pump? These pumps are used for returning seawater from a sump to the display aquarium. They are used in fish only aquariums and reef aquariums, provided the system includes a sump of course.
Equipment nowadays is generally very reliable and many aquarists don’t back-up anything. Fair enough, but any item of equipment however good can fail.
General good practice suggests that important equipment in the system should be backed up where possible. This is clearly not done in the case of lighting and display aquarium circulation pumps. Important as these are it is impractical and unnecessary to have a back up. In the case of circulation pumps, the aquarist could have a spare in the cupboard, but the loss of some circulation for a short period is not a problem so this is unnecessary.
One of the most important areas in an aquarium is the bio-filtration. Some would argue that it is the most important area as without it the whole system will fail, that is the livestock will suffer or be lost. So if canister filtration is the method in use, two canister filters are a good idea in case on fails.
Anyway, to get back on track. The return pump. Seawater gets to the sump by means of gravity and an overflow in the display aquarium. It flows through the sump and is pumped up again. Looking at the pump and its job, is it important enough to require a backup and if so is it practical?
One of the benefits of a sump is that it can house items such as a protein skimmer, heaters and possibly a deep sand bed (DSB). This being the case it would be detrimental for seawater not to be exposed to the protein skimmer for a long period of time. Similarly, the seawater needs to be maintained at the proper temperature. So the flow through the sump needs to be reliable. So a back-up is desirable.
One of the items that the aquarist should check when at the aquarium is flow, and a lack of flow from the display aquarium to the sump is very noticeable. Seawater loses heat slowly (the loss rate being subject to circumstances) so heat loss is not of great concern, the loss of flow should be noticed before any problem arises. The lack of flow will not be noticeable on the protein skimmer as it will continue to function though will fail to remove organics as they are not going through the bubble chamber. Any DSB will not be affected as seawater is present, though benefits such as nitrate reduction could temporarily be interrupted.
From a practical point of view, a back-up return pump is best considered in the sump design stage. Many sumps have a sectioned off area that is capable of housing one pump only. If two are to be used, the sectioned off area will need to be that much larger, unless the pumps can be fitted one above the other.
If two pumps are to be used, should they be of equal pumping capacity? The guideline for the flow rate through the sump is around three times the net gallonage of the whole system, that is display aquarium and sump, per hour. So two pumps together need to provide this, each pump having one half of the pumping capacity of a lone pump, that is, each pump needs to be able to deliver around one half of the required flow rate. Both pumps are very unlikely to fail at the same time, and the failure of one pump means that the flow rate through the sump will be half of that intended. This will be enough to maintain heat distribution, and will continue to present organics to the protein skimmer. The DSB will also continue to function.
If the aquarist does not check flow rates when feeding or admiring the display aquarium, or uses an automatic feeder and doesn’t check the aquarium particularly regularly, then having two return pumps is a good idea. If checks are regularly made and the aquarist is confident that a changed flow rate, or lack of one, will be fairly quickly noticed, then two return pumps are not really necessary. Consideration needs to be given however to how quickly a replacement pump can be obtained should this be required.
The individual cost of pumps where two are used will be a little lower because of the lower pumping capacity, so the extra cost over one pump is not great. It is a good idea overall to protect important system functions where practical, and the use of two return pumps is good and follows this principle.