Lobelia Cardinlais: Leaves curling downward

I have some Lobelia Cardinalis (Large form) in my 4-feet tank and
its leaves are beginning to curl downward and inward. The whole plants
look healthy, leaves look green. I couldn't be sure if this is a sign of
nutrient deficiency. Other plants are doing OK. I have more than
200 watt of CF lighting and regular dose of Tetra & Seachem ferts every
week at H20 change. I also have a combo of dennerle, JBL & root tablet
underneath my fine sand substrate.

Can anybody help to enlighten a bit here?

a.c.