How much light is too much?

10.1 wpg for a 75?

 

you could bleach your coral and get the water too hot at some point

 

What kind of lights?

 

2x 250w metal halides, 2x 130w actinics.
lots of ppl use this set up, im just wondering what the minimum size tank should be used with it

 

That does seem to be an awful lot of light. On a 75 I was told by many to go with no bigger than 175 watt MH light due to heat problems.
But as usual this is one of those subjects the if you ask 10 people you will get 10 different answers. So basically if it working for you then don't go with it.

 

on my 75g @ 21" high, i have 2-250w mh with 4-65w pc. pick the right bulb to match the par level dependant on the corals housed, and you will have a happy fast growing tank.