[APD] re: High water evaporation in a planted aquarium

[David Aiken <daiken at iprimus_com.au>]

The evaporation from your tank is pure H2O - the dissolved substances 
in the water do not evaporate but remain in the tank so hardness, 
nitrogen waste content, etc all increase slightly with evaporation. You 
don't have to worry about the nitrogen wastes with lots of plants and 
an efficient filter but when you add more water to replace evaporation, 
you're adding more hardness compounds and other things in your tap 
water and their concentration increases again. Keep repeating this and 
the nature of the water in your tank will change incredibly.

The ultimate example: consider your tank as the sea and the top up 
water as the rivers running into it, because that's the ultimate end 
point on this process. Water evaporates from the sea taking nothing 
with it and leaving the salts behind, rains down on the continents and 
islands, absorbing some contaminants from the air, dissolves and builds 
up a mineral content as it flows back to the sea where it deposits the 
mineral content as it evaporates again to repeat the process.

Water changes are essential to replace some elements absorbed by the 
plants, but they're also essential to help keep the overall mineral and 
salt content of the water relatively stable and in the freshwater 
ranges that we like. It probably would take a few decades or centuries 
of top ups only, without water changes, to get your tank to the point 
where it approximated a brackish water or marine tank but in theory at 
least you could do it.

David Aiken

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