[BryceM]

Alternanthera will be red under any conditions. I've never seen it otherwise. L. aromatica and P. stellatus will turn red under high light and good iron. When it doesn't color up nicely it's usually the light level IME. Your light seems sufficient, assuming newer bulbs and good reflectors. You might try increasing your micros.

How much are you currently adding? If I was using Flourish, I'd add 60ml per week and maybe 30ml of Flourish Iron on top of that.

 

[BryceM]

With that much light I don't think skimping on the macros is a good idea. I also seem to get more GSA algae with low PO4 levels.

 

[BryceM]

Most fish will tolerate nitrate levels up to 80 or 100 ppm just fine. I've also seen fish doing fine in tanks with 10ppm of PO4, but those tanks frequently had green water issues. I would recommend aiming for somewhere around 20ppm NO3 and 2ppm PO4. Toxic levels of these two macros are quite a bit higher than this.