if i was the OP i would be worrying about the things we can test... eg KH GH TDS CA MG
in a planted tank the plants/fish will use these up, so no you cant keep a tank and not do water changes and not add kh/gh buffers,
there will be evaporation, if you don't top it up with ro/di water you will be stacking your tds
remember the KH/PH correlation (when ur kh crashes your ph crashes) this will be inevitable in a planted tank without water changes,
in a tank without water changes even if the plant load can absorb the nitrate levels and keep them in check after a while they will have a CA deficiency and this will be a limiting factor in the growth rate of plants, even if its not the GH/CA as soon as you hit 0KH your plants will start melting back and there ability to uptake other nutrients will cease to exist as they are basically crippled,
of course if you add some buffers and top up with ro water it can be done, for a plant only tank but for fish i wouldn't do it unless you want some small/stunted fish
edit:if you want extra tannins you can add, Candian Peat Moss, IAL, Dried Bannana Leafs(purchased from aquotix) etc if you dont like them in your tank, you can soak them in the water change water whilst it ages 24-48hrs IAL will release alot
Edited by Bombshocked, 17 October 2014 - 03:02 AM.