I agree with Craig to increase the amount of P you are dosing into your tank.
However, even more than P, I think you are underdosing iron. Seachem's Trace is not a source of iron, it has very, very minute amounts of some trace elements, but not iron. Seachem's Florish has some iron (0.32%). Are you really dosing more Trace (15m) and less of the Florish (5m), and just two times per week (Mondays and Wednesdays)??? I think this is far too little and it may actually be iron deficency which is limiting your tank's growth.
If you would use the E.I. dosing guide for tanks 60g-80g and dose 1/4 teaspoon of CSM+B, three times per week, you would be adding 0.45 ppm iron into a 75 gallon tank weekly. If you're only dosing 10ml of Flourish weekly you're only adding 0.11 ppm of iron into your 75 gallon tank. From my experience, really not enough with strong lighting and good CO2.
I've gradually tweaked the dosing on my 75 gallon tank using the E.I. rates as a starting point. I now dose both Macros and Micros daily at the same time (morning, before lights on). I've increased the recommended amounts of P until I get no more GSA, and I've reduced the amounts of K because of what I get in the (KH2P04) P.
More importantly I dose around 1.5x the amounts of traces outlined in E.I, and I have no trouble with my fish. I've got Neon tetras, Congo tetras, and probably the most sensitive of the bunch: Checkerboard Cichlids (Dicrossus filamentosus), all looking absolutely gorgeous and I presume doing well. The only unfortunate thing is I've seven male checkerboards, and no girls