Whilst I won't question the effect of liquid plant fertilisers on the general hardness of the water, (and even pH), I would strongly advise against using these as a means to manage water hardness.
Liquid fertilisers are intended to feed plants and contain the nutrient minerals in order to do so. Most of the literature regarding these recommends not overdosing, as this can have a detrimental effect on the resident fish and can promote algae growth.
When setting up my tanks and relying upon good plant growth to facilitate my cycling, I happily used the Tropica liquid plant food. I might also use it when trying to establish new plants. However, I now choose to let my fish waste feed the plants and so do not regularly use a liquid fertiliser, for fear of the issues I mentioned earlier.
Also, understand that if you start messing with your water's hardness, then you are committing yourself to have to maintain those levels for as long as you run the tank and will need to make adjustments to every bit of water you add to the tank within its lifetime.