Thanks for the reply. I don't think this is BBA, as mentioned by the previous user, this is brown algae. For the past week or so, I started making frequent water changes and there is some effect on this diatom algae. The nitrate level was a bit high, albeit well within what API instruction says (not sure if this is too much for my tank).
Here are the tank settings:
Tank Size: 45Gallon
# Fishes: 18, 10 serpae tetras, 5 Kerri blue, 1 Angel and 2 German blue rams
Cleanup Crew: 5 nerite snails and 4 otocynclus
Beamworks Beamswork DA 6500K 0.50W,
Co2 - running about 1 and half bubbles per second.
Gravel - floramax planted aquarium substrated. Not a great one I guess, got it from petsmart.
Filter -> fluval aqua clear
Water changes mostly once a week. If there is increased algae, I do twice a week.
Plants - Staurogyne repens (srepens - for carpet), Amazon sword, Java fern, monte carlo (Micranthemum), rotala Hra ('Vietnam H’ra’) and Egeria Densa.
5-10 ppm nitrate
Ammonia 0-0.25 ppm
Nitrite - 0ppm
Nitrate - 5-10ppm
Ph - 7.6
GH - 50-100ppm
KH - 0-50ppm