It always surprises me how much water actually evaporates from a [tag-tec]saltwater aquarium[/tag-tec]. It does depend upon the aquarium of course, how much water movement there is on the water surface, if overflows are used, the type of lighting selected etc.
As we know stabilty is required in a saltwater aquarium. Stability of salinity again is very important. When the water evaporates the salt is left behind and the [tag-ice]specific gravity[/tag-ice] can slowly rise. If water is not topped up then the specific gravity reading of the aquarium when checked will be wrong.
If you do not use overflows and use fluorescent tubes for lighting then probably you will be able to maintain control of the water top ups required due to evaporation. It could be twice a week, it could be once a week who knows – but you might be able to stay on top of it.
On the other hand though if you do use overflows, metal halide lighting etc then probably the aquarium is going to require more water top ups than the aquariums that do not – I know mine does!
On my aquarium I use an automated water top up device. This is controlled by a float valve which is installed into the sump and is hooked up to the reverse osmosis unit. When the float device detects that the water level has dropped in the sump due to evaporation the switch opens and water is allowed to flow through the reverse osmosis unit into the aquarium, therefore doing the water top ups for me.
When I prepare for a water change I lift the float valve out of the water to activate the reverse osmosis unit, move the reverse osmosis feed pipe into the water change bucket and allow it to fill. When it is full I put the feed pipe back into position as well as the float valve.
On my fathers aquarium, however he does does not use overflows and uses fluorescent tubes therefore the water evaporation is very low. He manages to maintain excellent stability in his aquarium and maintains stability of salinity by doing manual water top ups and I have to say that his [tag-self]reef tank[/tag-self] has to be one of the best I have seen.
What I like about auto top up device is that the top ups are performed when I am at work, away on holiday and even asleep.
Of course you do not need to rush out and get one. If your aquarium does not require a large amount of water top ups then you will probably be able to maintain it yourself, however if you do experience a large amount of evaporation and have to top up with a large amount of water then possibly one of these devices is for you.
I know that it has worked for me.