I use 'pills/capsules', the ones you get with medicine inside that dissolve in your stomach.
I like them, you put them in once a month or 2 months, they have a 15cm advertised 'radius'.
Noticable on the plants in combination with added liquid media. I'd recommend it for sure. There's two types, stem and leave.
Not 100% sure, but they'd compare to the root tabs I think. Cant give you comparison though.
Brand I use is Ferka.
below a part of an email from them:
Aquatilizer is compose of all trace elements together with abundant potassium and also little nitrogen( not nitrate ). Somehow for serious tank keepers or one who need fourish plants, Balnace-K should be added.
Iron in Aquatilizer is enough to be absorbed by leaf.
Stemma is acomplete formular to be absorbed by root. By giving element into substrate which can be much more concentrate so we can expect for fast growing together with branching while less problem occur compare to liquid fertilizer and if you have rosette plant like echinodorus or nymphae, Rosetta is recommended.
Both Stemma and Rosetta have abunsant Iron in it.
End of quote....
Look up Ferka on the web, and find some of their display-tanks. Also, they've got a restaurant here in Bangkok with dozens of planted tanks, maintained with Ferka products.
my 2 cents...
cheers.