What you could do would be to add aquariumfertilizer.com Macro/Micro mix w/ CSM+B, and continue with using Flourish Comprehensive. What this will do is provide the necessary N-P-K macro's as well as supply micro's. The Macro/Micro mix is basically macro and micro fertilizers mixed together with added boron, and then using Flourish Comprehensive will make sure your plants are getting the nutrients they need. Plus depending on your substrate, it will also add in enough extra nutrients for it to soak up and later release back to the plants.
15g Fluval Flex - Will be a planted shrimp tank