I would not be afraid to bet, with proper kelvin ratings on the bulbs
you could get by with two or even one bulb @ 40Watts each on a 55 gal
tank without a problem..provided they are not too far off the surface
of the water. For proper light dispersal, the reflectors really need
to be polished metal (alum or stainless).......If I was a gambling
individual, I would bet your lights are your main problem in regards
to plant growth.