No matter how big or small an aquarium is they all have a gallonage which is often stated in the manufacturer’s documents. Some aquariums are of a standard size and are known as a ‘fifty’ or whatever.
The gallonage quoted in the aquarium name or manufacturer’s documents is with it empty, it has to be this way as of course the manufacturer has no idea of what use the aquarium will be put to and what will be inside it.
As far as the marine hobby is concerned, the quoted gross gallonage can be ignored, it is irrelevant. What is of interest is the net gallonage, the amount of seawater the aquarium can hold once all the decorative whatnots are installed. In the case of both a fish only and reef aquarium, whether live or inert rock is being used doesn’t matter; it is the amount that is installed that does. In the first place, if live rock is being used there must be enough to provide adequate bio-filtration for the full bio load. Secondly, the more there is of either rock type the less the seawater gallonage will be.
If the aquarist decides to use a DSB (deep sand bed) or plenum then this will again reduce the seawater gallonage. A decorative sand bed is not as deep but still accounts for lost seawater space.
So it is important that the aquarist is aware of the net gallonage of the aquarium. Having a sump has advantages and one of these is that the net gallonage is increased.
Calculating the net gallonage could be a hit and miss affair. Probably the most accurate way is for the aquarist to measure the prepared seawater as it goes into the aquarium on the first fill. However, many aquarists mix the initial seawater in the aquarium, but nevertheless a note could be made of the amount that goes in. If any of this first fill is eventually removed because of overfill it needs to be deducted of course. There are helpful gallonage calculators on some marine forums for those who did not measure the ingoing seawater, though accuracy will be lower.
Both the reef and fish only aquarium rely on net gallonage for stocking levels. This is for fish; corals present a much lower bio load. The reef aquarium carries less fish than a fish only system which gives regard to necessary seawater quality. With both systems seawater quality is the number one requirement – how can the aquarist stock correctly if the net gallonage is not known?
When calculating stocking levels for whatever system, the sump seawater should be ignored. The seawater in the sump is not available to the fish and should be viewed as a quality enhancement advantage.
Stocking to the gross gallonage of the display aquarium, or to an overestimated net gallonage could lead to trouble. The aquarium could be and is likely to be overstocked. If there are too many fish there is greater pressure on the seawater quality. In addition it creates more work for the bio-filtration. Further, if there was a temperature increase there could be oxygen problems. The fish obviously require sufficient oxygen, and so does the bio-filter, or rather all the oxygen hungry bacteria in it. If there are problems with the bio-filtration then real trouble could begin.
Also, overstocking can mean overcrowding. Fish will argue and even fight over the right to territory, some more than others. This could lead to fish not eating properly because of stress. On the wild reef fish have plenty of hiding places and in addition if they need to retreat there is plenty of space to do so. Within the strict confines of an aquarium where does a fleeing fish go?
So it is certainly to the advantage of the aquarist and his/her future enjoyment of the hobby to go through a careful initial procedure. This includes research into compatibility with tank mates and aquarium type of course, but should also consider the space that is going to be available. During reef or aquascape construction the aquarist should give full regard to supplying sufficient homes for the number of fish it is intended to keep, many commonly used rocks are helpful with this because they are of very irregular shapes. It will pay off as fish that are settled and secure will be healthy and show the colours that nature intended.