OK, I've had a look and these are the stats you posted if I'm not mistaken:
TANK
48x24x21
LIGHTING
luminare 2x150 6500k halides on 3pm-8.30pm=5 1/2hrs
2x54w t5 6500k tubes on 1pm-10pm=9hrs
pressurised co2 2bps
gh-4
kh-2.5-3
ph-6.4
co2-around 30ppm
FERTS
3/4 tsp kno3 3x wk
po4 10ml 3x wk mixed coz cant measure small enough amount
trace 50ml 3x wk=3tsp in 250ml water
w/c 50%+ wk.
PLANTS
Glosso
e.parvula
blyxa japonica
vallis nana
HC.
Well, my first impression is: "Wow, that's an awful lot of light". 400 watts on an 80 gallon tank is really serious but the exceptional flow you have seems to be working. Then again, all the plants except for the taller ones on the periphery look a bit yellow but I'm uncertain whether that is due to the photography or not. I'm a nutrient fanatic so I would say your KNO3 is marginally OK but you are way low on the PO4. I'm astonished to see the statement "Can't measure small enough amount" of PO4 because this tank can and should easily swallow 1/4 teaspoon KH2PO4 3X per week (and beg for more), no question mate. It should also be getting at least 1/4 teaspoon trace 2X per week. You could probably do with some Epsom Salts as well but I would look at the macro dosing first and foremost. The blotches on the leaves could easily be GSA (Green Spot Algae) and is a result of low phosphates.
Additionally, 2 bps sounds low on this size tank. It's not clear how you are determining the CO2 concentration. Are you just measuring the pH of the tank water and then using the pH/KH/CO2 table to derive CO2 concentration? If so you could be way off because other acids in the water can lower the pH giving you a false high CO2 calculation. I would suggest to look at getting a dropchecker with 4dKH water to give you a better idea. That browning you mentioned could well be the onset of CO2 starvation.
Cheers,