For those interested in CO2 levels and bioavailability this chart explains one of possible reasons why some plants grow better in lower KH.
When atmospheric or injected CO2 gas enters water it dissolves and some react with water to form carbonic acid H2CO3. This carbonic acid H2CO3 will dissociate to form bicarbonate HCO3. And then the bicarbonate HCO3 will dissociate further to carbonate CO3. However, these reactions are pH dependent. Water pH determines what form carbon will have. Since some plants can use carbon in bicarbonate HCO3 and carbonate CO3 forms most prefer dissolved CO2 and carbonic acid H2CO3. When we look at the chart, we can see the red line showing dissolved CO2 and carbonic acid H2CO3 forms rapidly decreasing 50% in pH of 6. The decrease of these preferred carbon forms is 87.5% between pH of 5.5 and 7.0. In other words, the same CO2 supply can have only 12.5% available carbon in the preferred form by just having the wrong pH. And this is why KH is so important because KH is affecting pH which dictates what form carbon will be in.
Also interesting is that 1 pH drop from pH of 7.5 to 6.5 has 3 x less carbon in the preferred form than the same pH drop from pH of 6.5 to 5.5.
One slight clarification: For water in equilibrium with some partial pressure of carbon dioxide gas, the amount of dissolved carbon dioxide is determined solely by the gaseous carbon dioxide partial pressure. pH doesn't actually affect it. However, at higher pH, you will have a lot more carbonic acid and bicarbonate, and if your plants can use these, they're better off. In other words, pH changes carbonic acid and bicarbonate relative to a fixed level of dissolved CO2.
Of course, if you're injecting CO2, you're not in equilibrium. You may not be in equilibrium even in a non-injected tank, if plants or fish are active enough. And this is a good thing: If your tank was actually in equilibrium with pure CO2 at atmospheric pressure, which is the ultimate result of perfectly efficient injection of massive quantities of CO2, your pH would be floored and your fish would be floating belly-up on top of your tank.