Factors that affect the reproduction rate of CRS

Dear all,

Need your advice on possible factors that affect the reproduction rate of CRS.

I have a heavily planted tank with around 50 pcs of CRS. Very few offsprings for the past six months. The following are my setup.

Tank: 4ft by 1.5 ft
Substrate: ADA Amazonia (approximately 1.5 inches thick)
Plants: Seaweed (Coral Pelia ?? not sure of the name) Covering 90% of the tank's floor and backwall
Food: Pellet food from Shirakura (feed twice daily)
Chiller 24.5 degree
2 sponge filters (air pump - quite strong current)
CO2 approximately 30 bubbles per min
PH - 6.2 - 6.7
2 T5 lights switch on 6 to 8 hours per day
2x2028 Eheim filters (sponge, ceramic rings and eheim substrates)
White Powder and Microorganism powder added daily
Liquid Minerals added weekly.
20% Partial Water change weekly(Aged water)

There are no casualties in my tank but they are not productive.

Hope you can enlighten me of factors that may affect the production rate

Thanks
Regards
Nbs