When it comes to figuring out how much flow to run through a sump system there are many different aspects to consider. It seems like most people fall into one of two categories. One camp of people think that a low flow of around 4-6X turnover is all that is needed to keep a constant supply of dirty tank water in your sump for filtration and heating. While the other camp says that a higher flow 10-15X turnover is the only way to go. When asked what I think, my answer is always, “it depends”.
While it is obviously possible to have a very successful reef using either methodology, it should be understood that either one comes with its own set of advantages and disadvantages. It is up to you as a hobbyist to understand them and to decide which one will benefit your specific set of circumstances the best.
When people are asked why they prefer to have a slower flow system, they usually cite 4 benefits:
1. Decreased Electrical Usage
2. Decreased Heat
3. Decreased Noise
4. Lower Purchase Price for a Smaller Pump
When people are asked why they prefer to have a higher flow system, they usually cite 3 benefits:
1. Increased Surface Skimming
2. Increased Mechanical Filtration (In-Sump Filter Socks or Floss)
3. Decrease Dead Zones in the Sump (Less Detritus Settling)
Before we go any further I think I should mention that I typically prefer to use higher flow systems in my situation. However, my situation may differ from others, and I can think of several instances where if my situation was different, I would consider high flow systems to not be a viable option. For example, I live in southern Texas where we have had temperatures in the triple digits for the past 2 weeks. We run our air conditioners in our homes for 10 months out of the year, and typically all keep our homes around 72 degrees. Our home temperatures do not fluctuate throughout the day. Literally, a home would not be habitable down here if our AC went out for even one day in the summer. With that said, my tanks have never run hot. Even when I ran two MH bulbs over a 120, I never had a chiller, and never had temperatures over 80 degrees. 2 hours after the lights turned off, my tank would begin to cycle the heaters throughout the night. So in my case, two of the advantages for the low flow people would be a moot point to me. Because 99% of the power being put into a submersible pump is eventually converted to heat. All the larger pump did was keep my heaters from running as long. If I have to choose between heat generated by water movement and heat produced from a heater, I’ll take friction heat from water movement any day of the week. However, if I kept my home at a higher temperature throughout the year and had to worry about my tank temperature during the middle of the day, I may not want to run a larger pump.
Noise created by our systems is definitely a very large consideration in every aspect of setting up a system. Everything from choosing where the tank will sit (don’t want it right by my head while I sleep), to how much money we want to spend to make sure your return pump is as quiet as possible. While an overflow can be designed to be nearly silent no matter what the flow rate, larger pumps will always make more noise, and while they are getting quieter, they still make some noise. This should always be a consideration when configuring a system. Especially if it is in a location that will be close to you while sleeping.
Performance is where the higher flow systems begin to shine. Obviously if you can increase the amount of water flowing into an overflow, the more proteins and particulates it will draw down into the sump. When running mechanical filtration, it can make an impressive difference when stirring the gravel or blowing detritus out of dead spots on rocks. The suspended particles are quickly drawn down to the sump and collected in the filter sock where they can be removed. Yes you can get the same effect over a greater length of time using a slower flow, but the odds of the suspended detritus finding another place to settle are higher than in a system where the water is filtered twice as many times in an hour.
Other than increased filter sock performance, the biggest advantage doesn’t have anything to do with the display. It has to do with your sump! When you have a fast moving flow through the sump, it radically cuts down on the amount of detritus that can settle in the bottom chambers of the sump. You want your detritus in one of two places. Either in the water column where your corals have a chance to consume them, or in a filter sock, so you can remove them from the system before the organics have a chance to be broken down into ammonia. Another option for lower flowing systems is to use powerheads strategically places in the sump to remove dead zones. This works great! However, you have to purchase several of these powerheads, which typically meets or exceeds the price difference of stepping up to a larger return pump. Also, they are another piece of equipment that can break, or at the very least, needs to be cleaned.
My final point has to do with sump design. Don’t get a 24” X 12” sump that holds a single 4” sock and expect to run 2000 GPH through it quietly or without introducing allot of bubbles back into the tank. When done right and sized properly, a sump should be relatively silent and should not introduce any bubbles back into the system. You mainly need to be aware of two things when designing or purchasing a sump. The first is baffle width. You can have a 6 foot long sump that holds a 100 gallons, but if it is only 12 inches wide, the water moving over the baffle will create noise and churn the water, possibly introducing air. You can have a 36” sump that is 30” wide and flow 1000s of GPH through it easily. In fact the water may flow so gently that it adheres to the side of the baffle, creating a nearly silent system that may not even require additional baffles to guard against bubbles. The second is the filter socks. If you decide that you want a system that utilizes filter socks, then make sure that is has an adequate size and number of socks to handle your flow rate. I typically prefer the 7” socks because a single one will flow allot more water before beginning to back up, but they do require more room. You can squeeze two 4” socks in a smaller space that will flow the same amount of water. However, you will have twice as many socks to clean each week.
If you are not concerned about heat issues, are comfortable with a little bit extra noise, and take the time to design a system for it, a higher flow rate may be the best choice for you. If you have your tank next to your bed, are sensitive to back ground noise, or keep your house warmer so that tank heat may become an issue, then a slower rate system may be your preferred method.
What do you guys think? What flow rates do you run your system at?
