No idea about the miracle grow brand but I used to use Jobes plant spikes, the fern ones I think. Used them for many years on two different tanks without a problem.
I now use Osmacote Plus which is a granular fert sold online and at some gardening centers. It's wildly popular for making DIY aquarium root tabs. All you need to do is pit a very small amount at the bottom of an ice cube tray and add enough water to cover them. Once they're frozen you can add them deep under the sand just like other root tabs. Be careful never to use to much and only use the osmocote PLUS. It will release ammonium which isn't very harmful but you don't want to much as that will be converted to nitrate and make it higher than usual. It's still the most often used and recommended product for making DIY root tabs.
