Maintaining stable parameters is something you will hear a huge amount of times in this hobby and one of the most important parameters is salinity.
The normal salinity range has a specific gravity of 1.026 or sits at around 35ppt (Parts Per Thousand) and this is prone to fluctuations as water evaporates from your aquarium.
In marine aquariums you don't really want to have lids that keep the water from evaporating because this limits oxygen exchange on the surface of the water and can cause you issues.
Therefore marine systems are best left as open top displays and this brings us back to evaporation.
As water evaporates from the aquarium only the pure fresh water is leaving the tank which leaves behind salt molecules and this makes the salinity rise every day.
All other parameters are affected slightly too but the main one in question here is salinity.
To combat this you need to add in RO or RO/DI water (Reverse Osmosis De-Ionised) to replace what was lost in evaporation.
Tap water, even if a conditioner is used will not be suitable because it contains many contaminants that a de-chlorinator or water conditioner will not make safe for use in a reef aquarium.
Even short term use of tap water can have huge adverse effects on corals and cause algae blooms.
To ensure the stability of salinity levels it's recommended to top up any evaporated water at least daily, some larger systems can loose more than 10L of water per day so you will need to have a good supply of RO/DI.
Certain times of the year can affect the rate of evaporation more than others, for example winter is typically less humid and the dry air tends to suck water from the tank much faster than it will on a rainy day.
More stability is achieved by using an ATO (Auto Top Off) these are devices can be mechanical or electrical and provide you with peace of mind that your salinity and evaporation are under better control.
Simple units like a float valve ATO are tried and tested for many years and they stand the test of time.
As the water level drops the float drops and opens the valve allowing fresh water to drip in.
Their down side is that the reservoir of RO/DI is gravity fed so you need to have a bucket or something in an obvious spot unless you get tricky with plumbing.
Other units have a float switch that activates a small pump inside your ATO reservoir which pumps fresh water back to the system until the float switch rises to the set level and shuts off the reservoir pump.
Newer units have an optical sensor, usually attached to the glass with a magnet on the other side.
It's uses light waves to determine if the internal microprocessor needs to activate the reservoir pump.
In a pinch you can use distilled water from the supermarket but make sure its fragrance free (ironing water), RO/DI is best and when dealing with a marine aquarium you need to give it the best possible care you can.
Also whatever ATO unit you use or if you decide manual top up is for you, it's essential that you have an accurately calibrated Refractometer to correctly measure the salinity of your aquarium.
Don't guess and don't use a hydrometer.
Information provided by:
MARINE AQUARIUM FANATICS AUSTRALIA
