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

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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