I'm not sure if they would be harmful per se, but the nutrients required for terrestrial garden/house plants (for which these products were designed) are inherently different than those needed by aquatic species or terrestrials in an aquatic stage. Therefore you will likely run into issues. The doses of nutrients required for terrestrial plants are generally greater, largely because they are not in an enclosed system- many of the nutrients are lost through the soil and by water washing them away. They'll also be able to absorb more from superior access to CO2. With a fish tank, it's a closed system- everything you put into it stays there, until you remove it (taking out plant cuttings, water changes etc). Aquatic plant fertilizers are specialized to provide only what the plants can use up, and I feel that a terrestrial grade fertilizer would offer a surplus of nutrients. These nutrients behave differently from each other, so would remain in the water, get absorbed by the plants, be lost into the atmosphere or be converted into other forms of the element at different rates, making it difficult, if not impossible to simply does the tank and leave it. If you were to use this, I would recommend doing a large water change each time you dose, and several in between dosing.
Just to be clear, none of this is factually or scientifically based, it's just my non-backed assumptions, so should be viewed as pseudoscience until proven otherwise... and now that I think about it, I knew of a guy who had a fantastic paludarium setup with healthy underwater plants who dosed miracle gro fertilizer. So go ahead, experiment, see where it gets you! You'll never know for sure until you try it.