Replacing Evaporated Water

Q) Why does my marine aquarium appear to 'lose' water? Is it likely to be harmful to the fish and invertebrates? How can I replace it safely?

A) Any body of water left open to the atmosphere will evaporate if not replaced. This can be demonstrated by wetting the back of your hand. Within a minute or so, it will be completely dry as the water quickly evaporates. The same principle applies to the aquarium; given long enough, a full aquarium would dry out completely! Realistically, the marinist would attend to any lost water as required by various methods, as we shall see. It is important to note at this point that it is only water molecules that evaporate, molecules of other substances remain in the main body of water, effectively making it denser. The end result being that specific gravity (s.g.) and salinity start to rise if measures are not taken to redress the balance. Both fish and invertebrates are sensitive to changes in these parameters as it affects their osmoregulatory systems markedly, leading to stress and possibly disease. Therefore, it is in everyone's interests to replace water as soon as it evaporates. (Unfortunately, a complete explanation of the highly interesting, but nevertheless substantial topic of osmoregulation in fish will have to be the subject of a future article).

Replacing Evaporated Water 
There are three main methods for replacing evaporated water (most are referred to as osmolators) : 
  1. Reservoir above the tankSplosh it in! This is the crudest and potentially most harmful procedures. Basically, it entails pouring in a jug or so of water as soon as the level has perceived to have dropped. To be fair, by marking the side of the aquarium and measuring the shortfall accurately, a daily replacement would be just about acceptable. However, should evaporated water go unnoticed and the density of the aquarium water be allowed to increase, livestock will be in for a sudden shock when it is replaced! In this instance, large quantities of top-up water must be allowed to drip back into the aquarium via an air line and valve from a suitable reservior above the tank.
  2. Semi-automatic reservoirSemi-automatic. This technique relies on our old friend atmospheric pressure and enables a top-up system that is very acceptable. A sealed box (usually acrylic or glass) is placed on the strengthening bars just above the surface of the tank water. The only entrance to the box is a pipe that protrudes from the bottom of the container and extends to just below the surface of the water. Water can only exit from the box when the main tank water falls below the level of the pipe e.g. when it evaporates. At all other times, atmospheric pressure keep the liquid safely within the box. It is essential that, other than the outlet/refill pipe, the box is absolutely sealed; otherwise water will escape unrestricted.
  3. Fully automatic reserviorFully automatic. Such a system is a boon to the marine aquarist. Water can be replaced as it is lost (sometimes to the millilitre!) and a larger freshwater reservoir can be utilised, thus enabling maintenance to be kept to a minimum. A sensor is positioned at the required level in the main tank or sump and informs an electronic switching device that the level has dropped. This device then engages a pump, usually situated within a freshwater reservoir, and tops up the tank until the desired level is reached once more (the sensor detects this and disengages).
Some Tips

Always use distilled, purified (available from a pharmacist) or reverse osmosis water to top up an aquarium. Never use tapwater or partially filtered water as any impurities will just continue to accumulate, making the water more and more dense (remember, only water molecules evaporate). Gradual dosing with additives can effectively be achieved by mixing them with the top-up water in the semi and fully automatic systems. The additives should preferably be of the kind that remain in solution e.g. calcium chloride. When performing water changes, don't forget to remove or switch off the osmolators; otherwise they will dump all their contents into the system when the level drops. Never use such a large water reservoir that, if the sensor remains switched on accidentally, the whole contents will overflow the tank onto the floor! Clean sensors regularly, especially if the tank suffers from nuisance algae.

© Nick Dakin. May not be reproduced in part, or whole, without permission.