I've been using only
Flourish Comprehensive and root tabs for well over a year now, and have several well-planted low-tech tanks as a result! My PetSmart carries both, but if you can't find it locally Amazon carries them as well. I also have three little African Dwarf Frogs - again, for over a year now. They really appreciate their planted tank, and have never shown any signs of having issues with the ferts!
As BDM said, it's a company - they're going to try to up-sell their products!
You *could* put additional ferts into your system, but they aren't necessary in your situation. With any planted tank, a balance has to be found between Co2 (naturally occurring in a low tech tank), lighting, and fertilizers - this way the plants will be able to grow as well as they are able, given the light they have - and uptake ALL
of the nutrients provided by the ferts/fish. We've all found a good balance with Flourish Comp, root tabs, and lower lighting. Unless you have high light, and are adding Co2 (high tech tank) you'll likely cause more harm than good by adding more ferts than the plants can consume - anything left over after the plants are done for the day feeds algae. . . and is a waste of money!
Hope this helps!