First I was good, and now I suck.

i was under the impression that although new plants often melt or take time to acclimate to a new fish tank, they eventually start flourishing. However, the impressive growth you see in the beginning isn't due to the conditions in the new tank but built up reserves from where they were grown. Therefore, the hard part isn't getting plants to survive but to maintain growth. I'm guessing your plants grew initially well but eventually ran out of nutrients and died. I'm probably wrong though...
 
Without dosing nutrients there's a hole in the chain. Excel is basically liquid carbon, but you still need your macro's (NPK) and micro's. More likely than not you aren't getting enough from tap levels and fish bi products alone.
 
2 weeks between wc's is a long time.

I have gone 2 month before with no problems.
I didn't have plants in the tank though..
 
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