Here are my thoughts
1. You may consider getting a PAR meter rather than a lux meter
The Apogee Instruments' Model QMSW-SS (with optional BNC connector) has been recommended but I think they have now changed their model numbers...you'll need to email them for info.
Apogee Instruments
2. Don't try a one size fits all solution - perhaps move the ones you feel are coloring well to ambient light into a seperate tub or area. Those you think need supplemental light place them elsewhere and provide the light.
3. Light rails appear to be a good idea...there is I beleive a thread on this..Hahnmeister has installed this system on his private tank. I wish I could do it but my canopy is too low.
4. You've got a wonderful opportunity with the PAR meter to do a bit of research and advise the rest of us as well as I guess your customers.