Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Tap water that does not have any real problems, just too high GH and KH, can be used in a blend with RO. If the water has chlorine or chloramine you will need to dose dechlor for this.
RO has close to nothing in it but water. (Test it to be sure. TDS meter is really handy)
Plants need 16-17 different elements to grow.
GH measures Ca and Mg, so if the GH is roughly 3 degrees or higher there is plenty for plants.
KH measures carbonates. This is a buffer for pH. The old thinking was that you needed at least 3 dKH for a stable pH. I have stable tanks with no KH showing on my tests. (No, the test is not old)
If you can get the RO + tap blend to the GH of 6 and KH of 2, I think that should be fine for the plants, and OK for most soft water fish. Even less GH is OK, but you might want to boost the KH, if you cut it much lower. Nitrifying bacteria use the carbonates as a source of carbon.
All the other minerals that plants need are usually included with a trace mineral supplement, so if you are using some sort of supplement like SCM+B, or Seachem Flourish Comprehensive (there are others, too) the plants will be fine. You can add some when you do water changes, then a bit more through the week to keep the level up. A substrate with good cationic exchange capacity will also hold a reserve of most of these minerals.
N, P and K will have to be dosed, also. Usually there are several sources.
Fish food supplies N and P, but not so much K. Enough N and P for a low tech tank. Probably still need to dose K.
A high tech tank will need to have all 3 added.
Slow release tablets can be added to the substrate to help out.
You can buy all the fertilizers dry and mix your own.
You can buy liquid fertilizers.