Easy Plant Fertalizer?

I wouldn't bother with a plant fertiliser. Especially with slow growing plants, your fish should provide all the  fertility you need. Adding fertiliser to the water will likely just encourage algae.
If your rooted plants grow too slowly, which they may do after a while, then you might need to use root tabs, but I wouldn't do anything until you see how things are going. The pond soil you added should provide plenty of fertility for a good while. I'm a great believer in not adding anything to your tank unless it really is necessary. I've never added any fertilisers to my tanks and the plants grow like crazy.