Slow Growing Plants

[RedHerring]

Hey all,

I've got a bit of a problem. I'm running an aeroponics system (it's a tote-bin with drip on the perlite and misting below and reservoir at the bottom) as well as a few potted plants as a control. Now I've got a green-pepper plant growing in the aeroponics system and a few other things in the potting soil but they're all growing incredibly slowly. Basically the get a pair of adult leaves then sit there doing nothing. They all look happy; good green leaves, nice and glossy with no spots or discolorations but they just don't seem to be getting bigger.
Here are a few measurements:

Air Temp: 10-22C
Water Temp: 18C
Humidity: 20 -30%
Light: 18h on 6h off. 6x 70W high pressure sodium + 1x 85W/5100lm "Daylight" fluorescent tube.
pH: 6.3
EC: 0.68 mS


Any ideas?