what ratio should we use to determine amount of mh light and no actinic.
will produuce usable light in a 2ft by 2ft squre (actually it's a circle, but you get the point), so it will provide light for 1ft in any direction from the bulb (horizontally speaking). To determine the wattatge you need, you have to consider how tall the tank is, and what your keeping. I would say 175 watt halides are good to about 16-18" in depth higher than that and the animals you can keep become limited. From 16-24" I would use 250W halides, anything over 24" I would use 400W and up. This would give you enough light to kep light demanding SPS
at just about any depth in the aquarium, however, if SPS
and light intensive corals don't interest you...you could up the scale a bit and use 175W upto about 20-22", still have lot's of light at the top, but at the bottom of the aquarium, you wouldn't have as much, etc.... So you can see there are many variables, as well as opinions. As for the NO light, you would use that for asthetics, use enough to make the light look good. If your using 6700K MH
(very yellow) you may need 2 or 3 NO actinic tubes, but if your using a higher K
bulb like a 10K, you may need very little supplamentation, if any, this will also be influenced by the manufacturer of the bulb, not all 10K bulbs look exactly the same.
Now with that said, what are the demensions of your 50g tank, and what are you wanting to keep?