[Edward]

This is going to be difficult because the KH is taken by plants in variable rate. More light, nutrients, and temperature pushes plants to take up more KH. You are going to be chasing the KH level every few days.

I found two possible solutions:
One is an automatic RO water flow via the tank. Hook up one extra filter between the RO line and the tank. Fill it up with dolomite sand and it will give your water constant KH , Ca and Mg. This flow will be reseting all levels back to normal. You can also use lime or coral to get KH and Ca.

Second one works if you have acid loving fish like Tetras, Altums and Discus. Let the KH drop to zero and never bother again. The pH will get stabilized at 4 to 5. Only one plant that doesn't grow without KH for me, is Echinodorus Tenellus. But the fish love it and other plants grow nice too.

Using baking soda to raise KH is not the greatest idea because the water will end up with lots of remaining Na making it difficult for plants to absorb some nutrients.

Edward

 

[Edward]

gpodio,

Baking soda is NaHCO3,
if Na accumulates over time then plants won't be able to take up nutrients at available low concentration levels. The nutrient dosing will have to be elevated to overcome this problem and therefore the algae grow may be also increased. Same theory applies to excessive K, Ca, ...
Na won't build up if water changes are done periodically. But then, not everyone enjoys wasting time pumping water all the time.
There are healthier alternatives to baking soda. Run your RO water over fine gravel CaHCO3 to get Ca and KH, or over dolomite CaMg(CO3)2 to get Ca, Mg and KH.

fishfry,

CO2 makes carbonic acid H2CO3 that takes pH down. Baking soda NaHCO3 makes KH and takes pH up.
Plants love Carbon, so they are looking for it in any available form. One is supplied as CO2 and one as baking soda NaHCO3. Most plants do prefer carbon from CO2 but there is always going to be some HCO3 uptake as well.

Not sure what the "KH+" liquid is made of but in most cases baking soda is used to increase the KH. Also, peat can remove KH from water, do you have any in the tank?

Edward