Maintaining stable parameters is a phrase you’ll hear a lot in this hobby, and one of the most critical factors is salinity.

The usual salinity range has a specific gravity of 1.026, or roughly 35ppt (Parts Per Thousand), and this is subject to variations as water evaporates from your aquarium.

Lids that prevent water from evaporating are undesirable in marine aquariums because they impede oxygen exchange on the water’s surface and can create problems.

As a result, maritime systems are best left as open-air displays, which takes us back to evaporation.

As water evaporates from the aquarium, only clean new water exits the tank, leaving behind salt molecules, causing the salinity to grow every day.

All other characteristics are influenced significantly as well, but salinity is the key one in consideration here.

To counteract this, add RO or RO/DI water (Reverse Osmosis De-Ionised) to restore what was lost through evaporation.

Even with a conditioner, tap water is not appropriate for use in a reef aquarium since it includes several toxins that a de-chlorinator or water conditioner would not remove.

Even short-term exposure to tap water can have a significant negative impact on corals and trigger algal blooms.

It is advised to fill up any evaporated water at least daily to guarantee the stability of salinity levels; some bigger systems can lose more than 10L of water each day, so you will need a significant supply of RO/DI.

Certain times of the year can have a greater impact on the rate of evaporation than others. For example, winter is often less humid, and the dry air tends to suck water from the tank faster than it would on a wet day.

Using an ATO (Auto Top Off) device, you may obtain more stability. These devices can be mechanical or electrical, and they provide you piece of mind that your salinity and evaporation are under better control.

Simple devices, such as a float valve ATO, have been tried and tested for many years and have been shown to be reliable.

As the water level falls, the float descends and the valve opens, enabling new water to enter.

The disadvantage is that the RO/DI reservoir is gravity fed, thus you must have a bucket or something visible in an obvious location unless you get creative with piping.

Other models include a float switch that starts a tiny pump inside your ATO reservoir, pumping new water back into the system until the float switch rises to the predetermined level and turns off the reservoir pump.

Newer models have an optical sensor, which is often connected to the glass with a magnet on the other side.

It employs light wavelengths to identify whether the reservoir pump should be activated by the inbuilt computer.

In a pinch, distilled water from the store may be used, but make sure it is scent-free (ironing water), RO/DI is preferable, and when dealing with a marine aquarium, you must provide the finest possible care.

Also, regardless of which ATO unit you choose or if you prefer manual top-up, you must have an adequately calibrated Refractometer to reliably detect the salinity of your aquarium.