Plant fertilizers are available as liquid or substrate fertilizers. Both should only contain the micro nutrients. Liquid fertilizers have to be dosed more frequently; substrate fertilizers will last longer. Since there are no obvious differences in efficiency, it is up to the aquarists' preference which to use.
Next to the micro nutrients, fertilizers contain chelates. The chelate is an organic molecule which binds metal ions thus protecting them from early precipitation. The preferred type is abbreviated DTPA because of its stability up to a pH level of 7.5
Unfortunately some fertilizers contain the chelate EDTA, which is much cheaper. However chelate EDTA is only stable at a pH up to 6.0 and therefore mostly useless in aquariums.
What do you think?