I've fought the same problem for a long time. Can keep fish just fine but plants were a challange.
I changed my substrate and mixed in the flourite...expensive so I went half and half. Hubby rewired my cheapo lights to overdrive them. You can just do a search on the net and find the directions. I added plant tabs into the substrate. I tried the flourish excel but it melted my vals....I didn't know that was why till I read that somewhere else.
Then just added a crap load of fish. In a 125 I have about 60 cardinals, 50 or so marble hatchets, 40-50 green neons, a few dozen pygmy cories, a dozen habrosus cories, 12 parotocinclus, 10 pencils. I plan on adding about 24 or so rummy nose tetras and maybe some red phantoms. That should fill it up.
I also picked out some easier to keep plants. I have marble queen swords, crypts, tons of anubias of different types, a few types of java jern including a trident that was given to me, and three different types of mosses.
Initially some of the plants up and melted away but all have come back. My biggest problem now is keeping the babies from the sword in check so they don't block the light from the lower plants. That and that darn duckweed that found it's way in there.