Topoff for tanks is vital, as pure water evaporates and needs to be replaced with pure water. Maintaining a constant water level will assist greatly in maintaining a constant level of salinity. This in turn has a major impact on the stability of the aquarium system in total.
Since evaporation only removes pure water vapour, the salt and other dissolved components are left in the tank. It is therefore important to only top up evaporative losses with pure water.
Replacing evaporation with salt water will result in significant increases in salinity level, which will be detrimental to marine organisms. However, this can be used in the short term to slowly increase the systems salinity if it is too low.
There are a number of different techniques that can be used to add water to a system to replace that which has evaporated.
Simply pour fresh water into the tank to the required level. This can create issues with large salinity changes between top ups, this is not a recommended way of doing things unless you add small amounts frequently. Particular care should be taken in small aquaria, where an addition of only a few litres of fresh water can have a significant effect on tank salinity.
Simple method of restricting the output from a container so that the drip rate is consistent with the evaporation rate. It must be noted that it may prove difficult to exactly match the top-off drip rate exactly to the rate of evaporation, and care should be taken to closely monitor the system and corrections made to compensate.
Automatic Gravity Fed
By using a float valve connected by siphon feed to a freshwater drum (reservoir), water will be topped up to the tank as it evaporates, maintaining the water level where you want it and maintaining salinity at a very constant level. All you have to do is make sure the reservoir has fresh water in it, and that the water level in the reservoir is located higher than the water level in the aquarium to maintain the siphon flow.
Float valves can be sourced from irrigation and plumbing supply stores, and are often used for animal trough or hydroponic top up devices. Be sure that the float valve is all plastic in construction, and does not contain any metal parts. RELN make a good all plastic unit. Another is brand is Stefani (available from Bunnings in the drinking water purification section).
These are similar to the gravity fed system, except they use an electronic float switch to control a low flowrate dosing pump. Peristaltic pumps are a good choice, as they are able to consistently deliver a very slow, measured flowrate, and are tolerant of running dry if the reservoir is emptied.
There are various designs and off the shelf units, of which the Tunze Ozmolator is one.
Dosing with Kalkwasser
Many of the top off methods outlined above can be used to dose kalkwasser, either by feeding the pumped top off water through a kalk mixer, or by pre-mixing the kalk and using that as your top off water.