My show tank is 126 gallons, but the water volume residing in the circulation pipes and the Jaubert and skimmer sumps brings the total water volume to roughly 200 gallons.
The greater the water volume, the less likely that bad things will happen quickly to your system. Changes in water chemistry, temperature, and even disease will occur more slowly in larger systems. Maintaining a "micro-reef" can be like balancing on the point of needle because imbalances can happen so quickly and can have a devastating effect before you can react. The more water (environment) that you can maintain, the greater the chance that you will notice problems before they become catastrophic.
Space is an issue when trying to maintain a large water volume. If everything has to fit under your aquarium, then you may have no choice but to consider your show tank the majority of the reef "environment". If you can pipe water elsewhere however, then you can increase your reef system volume without much trouble.
Out on the reef, the water changes continually. Currents bring in open ocean water and sweep away the waste products of reef-inhabitant respiration. The entire ocean is the "sump" for the reef. So how big should your sump be anyway? Just as some people say that you can't have too big a skimmer, I would venture to say that you can't have too big a sump. When something dies in on the reef, it is hardly noticed, but when something dies in your show tank, it can pollute your entire "ocean" in not even hours, but minutes.
The larger your system water volume, the more dilute pollutants from death and respiration will be, and the longer it will take for toxic levels to build. If you are watching your tank carefully (the best thing that you can do-- even better than water testing, though that you should do as well), then you will notice subtle changes in your animals that should alert you to the presence of a problem. So even though you'd like to, if you can't watch your aquarium all the time, then design it to have a large water volume. In doing so, you will be buying yourself time to react when problem occur. With a large water volume you should find that you don't need to do water changes as frequently, or if you do them regularly, they will be a smaller percentage of your total water volume. This is also good because water changes, for all their benefit, do represent a change in your reef environment and you should always seek stability in your reef system. No matter how hard you try, a water change will affect major parameters such as temperature, salinity, pH, and other minor water constituents including reductions in less desirable things such as ammonia, nitrites, and nitrates.
Right now my total water volume is only about 63% greater than the show tank volume. That's not bad, but I will be increasing that in the future (if not already by the time of this reading) with the addition of other home made systems such as a calcium reactor and my "well water chiller." Other than what resides in the circulation system, most of my additional water volume exists in either the Jaubert sump or the skimmer sump. These two sumps are separate and are connected by a two and one half inch PVC pipe.
The system is designed such that a power failure can not cause an overflow of the sumps or a draining of the show tank. The show tank has a surface overflow and when the main circulation pump stops, the inflow to the tank stops but the overflow quits as well since no siphons are present. Therefore in a power outage, the show tank remains completely full (the water level dropping perhaps only a centimeter.
In the basement, water that is in the circulation system drains back into the sumps. Both the Jaubert and skimmer sumps are designed with additional depth to accommodate the total volume of water contained within the circulation system, so a power outage can not result in an overflow. I have designed a float-operated beeper that alerts me when the sump levels rise. This is mainly a feature that would warn me of a failure in either the main circulation or skimmer pumps as both will result in a sump level rise (the reason that a failure of the skimmer pump results in a level rise is because it is a forced down-draft design similar to the commercial ETS skimmer ( my twin-tube design is as good, but hundreds of dollars less expensive!), and water is always suspended above the sump in the skimmer when it is functioning).
My Jaubert sump is a 40 gallon glass aquarium that I got for free from a friend who used it to raise bunnies. This is only about 60% full in operation, with the rest of the unused volume to accommodate circulation system drain-back. The skimmer sump is home made from dark 0.25 inch thick acrylic. I constructed it using Weld*on-40 cement for guaranteed strength rather than using the quicker methylene chloride solvent. I prefer using dark "smoke colored" acrylics for sump components because they are just transparent enough to see water levels, but they block diatom and algae-producing light.
Both sumps have dust covers which also serve to keep heat in and cut down on water evaporation. The Jaubert sump has a glass cover while I chose to use approximately 1,000 polyethylene half inch hollow spheres to cover the skimmer sump surface. This is an antievaporative technique used in the metal plating industry. It allows me free access to the water for taking samples or to add makeup water or chemicals without having to open and close a lid continually, the cumulative exposed water area between the double layer of polyethylene balls is probably less than one square foot.
The glass Jaubert sump has a hole drilled in one end at a height that could not allow the water level to drain below the height of the live sand bed. A plastic filter grate covers this hole. A two and one half inch PVC pipe then connects the Jaubert sump to the skimmer sump.
Each of these sumps is on a heavy duty shelf that is attached to a concrete wall in my basement "aquarium command center." Even though everything is on perfectly level platforms mounted against concrete, I didn't want to make any connections rigid for fear that foundation settling, an earthquake, nearby meteor impact, or nuclear explosion might cause a minor misalignment in the height of the two sumps. So rather than running the pipe between the sumps directly, a rubber coupler from the Home Depot was used at either end of the PVC pipe. These are water tight and are made to hold much more pressure than I am generating in equalizing the two to six inch head between the two sumps. These rubber couplers will easily allow for up to a half inch of misalignment before presenting any resistance.
Using these techniques, one can string sumps around where ever there is room. They don't even have to be at successively lower levels since flow will occur so long as the inflow from the show tank is at one end of the chain and the circulation pump is at the other.
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