Displacement – What Is It?

ArchimedesI think it was the Greek Archimedes who first discovered displacement. He was in the bath apparently and water flowed over the edges. After wondering for a while he called out “Eureka!” and leapt out of the bath. It might have been a bit of a surprise for anyone nearby.

To describe displacement – it is the quantity of a liquid that is displaced by a solid body when that solid body is placed in the liquid. The weight of the liquid displaced is equal to the reduction in weight of the solid body in the liquid.

So there we go, basically it is a very old world Greek who overflowed his bath and then ran around shouting about it.

Displacement is relevant to aquarists. When an aquarium is purchased it will be for a given gross volume but the aquarist cannot leave it at that. Whether the intention is to run a fish only aquarium or a reef aquarium, the actual volume of seawater – the net amount – will be less. This is because when the rock formation is placed in the aquarium it will displace seawater. The fact that the immersed rock is lighter by the weight of the seawater it has displaced is perhaps interesting but not really useful in this context.

With a fish only system the amount of rockwork is likely to be less than a reef (though not necessarily so). This is fortunate as fish only aquariums are stocked more heavily than reef aquariums – the amount of seawater displaced will be less, meaning there are more gallons for the fish. The reef aquarium usually has a sizeable rock structure so that the aquarist can place a considerable number of corals on it. This means more seawater is displaced, which in turn means there is less gallonage for any fish. This doesn’t really matter as the reef aquarist needs to have a considerably lighter fish load to protect seawater quality. This seawater quality is particularly required by the corals.

Another point to bear in mind is that when constructing the new aquarium it is fine to fill with seawater in advance, but not fully so. If the rock structure is inert, that is, not live rock, then it can be constructed before any seawater goes in. Once construction is completed the aquarium can be fully filled. However, if the rockwork will be done with seawater in the aquarium the usual level that the seawater reaches initially is around 2/3rds. This of course is to allow for displacement, once the rocks are all in the level can be brought up fully.

A good thing to do when keeping a notebook with the aquarium (highly recommended) is to note the amount of seawater that goes in. Note the amount that goes in initially and when the rocks are in note the final amount also. Add the two amounts together and that is the net gallonage of the display aquarium. If there is a sump the same thing can be done, although in this case it could be displacement by a deep sand bed (DSB), heaters etc. Add this to the display aquarium net amount and that is the net volume of the whole system.

Knowing the net volume is useful if medications or supplements are to be added. These are normally administered at X amount for Y gallons. It is also useful when stocking with fish in both reef and fish only systems.

When a new coral that is attached to a rock is added, make sure the seawater is at the correct level before putting the coral and rock in. When it is in, remove the excess seawater and note the amount. Remove this amount from the noted net volume of the system.

  1. You really did a good job on this article. I think your most important point was for aquarists to note how much water they add on their initial set up.

  2. Thanks very much for the compliment. Always good to hear!!

  3. Very informative article John, I’m a beginner in this topic, I know some people think it’s not really that important but this specific information might come in handy someday in the future. Thank you.

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