After reading you replies and other infos, I have been rethinking the whole thing and here is what I will try:
2X 250W DE 14.5k Megachrome (Giesemann)
using Sunlight Supply Reef Optix III+ reflectors and the PFO-HQI (ANSI -M80) magnetic ballast
4 (or 6?) T5 80W : half pure actinic and half actinic +
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