Hello Roger

Ingo will probably give you more competent answer, but here's my opinion:
Osram HQI-DE70WDX has somewhat higher luminous flux (average of 4300lm) and average rated life of 10 000 hours, while HQI-DE70NDX has luminous flux of 3800lm (mean value)and expected life of 8 000 hours. Another difference is colour temperature - HQI-DE70WDX has colour temp. of 3000K (which is "warmer", more yellowish light, while HQI-DE70NDX has colour temp. of 4000K (which gives impression of more natural light,"daylight". Both lamps have pretty decent colour rendering index (CRI) of around 80.
For the tank of your size 1 70W HQI lamp would be OK in combination with 3-4 18-20W fluorescent tubes. If the temperaure on hot spot is still to low, you might add one lower-wattage halogen spotlight (say, 35-50W).

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