When you go to the LFS, take a sample of tap water and ask them to check for ammonia.
How is your Red Foxtail doing? It's a 55(18" tall), with almost 3 watts/gal. My guess is that you will get some pink/red in Ludwegia and plants similar, especially as they get a bit taller. Just about any red/bronze varieties of Cryptocoryne will color up nicely for you at that wattage. I've got some Crypts. growing in a 55(2.0 watts/gal.) and in a 40(3.0 watts/gal.) that are bright red. Barclaya Longifolia is a beautiful red/pink plant in my 40. I just love that plant. Red Tiger Lotus will come up fairly nicely.
Nitrates play a big roll in the redness in plants. When the tank mavens want to show their tanks, they cut back on N for a week or so and the reds tend to come out more pronounced. I would not recommend messing with your N for affect at this time, however. Let's get the green slop out of it and get it balanced first.
If it were my tank, I would go with 25% tap the first week, and see how everyone responds. Then go from there with it.
I would dose no Fe, and shoot for P=1.0, N=1.0(you probably won't need to dose much with your fish load), and K=10 - 15. Since you will probably not be dosing a lot of KNO3, K2SO4 will bring that up for you. Mg is good.
What is your dosage of Flourish? In other words, how many mls. are you dosing per week?
Len