| | Re: Methods of softening water.
Hard water might slow growth of some sensitive plants, but for the most part plants will adjust to a fairly wide range of parameters. What they don't adjust well to is a constantly changing environment. Fighting your water chemistry is a good way to develop a situation in which you're walking a tightrope between consistant growth and catastrophic algae outbreaks. Over time your plants will soften the water on their own which could make growing sensitive plants a viable option unless your water is outrageously hard. However fighting your water chemistry requires a lot of work, so its a matter of whether you'd rather enjoy your aquariums without a high workload or constantly be fighting to keep them in check. I, for one, would rather work with my water than against it.
Water hardness has little effect on CO2(it is true there is a relationship between the two but it is a weak relationship that is easily overcome with a consistant CO2 source). The biggest factor hard water has on growth in and of itself is the interactions between Ca-Mg-SO4-K but anecdotes often overstate this relationship to a point of absurdity. For me personally, I'd rather see consistant healthy growth than super fast growth that requires a high degree of input on my end regardless. Especially since often fast growth aquariums are teetering on the brink of disaster if regiments aren't kept fairly strictly.
If you're willing to put in the work, using humic acids in your filtration will create Ca-Humates that will remove the mineral hardness(carbonate hardness will adjust itself accordingly over time mostly) but the harder your water the more peat/humus/etc will be required and the more often changing out the softening agent will be required.