Plant Food / Fert
Here's the scoop on plant ferts: Plants need two types of nutrients, macro and micro. The macro type are those the plants get with routine water changes and from the fish waste.
The micro nutrients you have to provide. There are several companies that make micro nutrients and there are different forms like, dry, liquids and tabs, etc. You can order these online or do what I do and that's get them from the local hydroponics store.
I like liquids because they're easy to use and currently use a couple of different kinds called Earth Juice "Grow" and "Microblast". There are a number of different kinds, just do a little research online and look for a good fert that has the following micro nutrients: iron, manganese, zinc, boron, copper, cobalt and molybdenum.
"Fear not, my young apprentice. Just change the tank water."