You're using a 70watt HQI over a 20gal tank?
I have a 15 gallon tall tank and I run 120watts of MH light over it with no problems. I also have a 5 gallon tank and run 100watts of MH over it. Before you flip your lid, read on.... :-)
In my case, I'm trying to target a specific amount of light on the bottom of the tank. I do this by measuring with a PAR meter (a meter that measures photosynthetic active radiation
) and I forumlate my lighting until I get the amount I want. I specifically target 100 µmoles/m2/sec because I consider this amount to be "moderate" light. It has been shown that plants saturate photosynthesis around 600 µmoles/m2/sec. And T.Barr used 450 µmoles/m2/sec when he did his research for the Estimative Index because he considered that to be very high light for aquatic plants. Also, Diana Walstad reports that 200 µmoles/m2/sec is a lighting intensity found in many natural places where aquatic plants are found growing in streams and lakes.
So, I guess the point I'm really trying to make is that even though one may use MH, VHO, or CF, it all depends on how much photosynthetic light is really getting into the tank. Watts per gallon doesn't tell you very much when evaluating a tank's lighting setup. That's why my 100W MH over my 5gallon doesn't compute for many people.
I really look forward to the day when we can all talk in terms of µmoles/m2/sec (PAR) because then it doesn't matter WHAT kind of lights you use as long as you're achieving a sufficient amount of PAR. And consider this: Even if you used CF and I used MH, 100 µmoles/m2/sec measured on the bottom of my tank would be the exactly
the same as 100 µmoles/m2/sec measured at the bottom of your tank! Wouldn't that be nice??