I live in North Florida and I have 200 gallon pond with 100 fish in it. Fish are doing fine and the plants are NOT thriving. They are barely staying alive.
Where is the system, how is the sun or lack there of?
Without any of this information it is really hard to diagnose your problem. My first stab will be that you probably have a high pH? And you will need to figure out why your pH is 8. Am I close to did I just pin the tail on the teacher?