lighting is a single pointn light source and since this is the case the further you are from the source emitting the light the less light will hit your tank. 90gal tanks are 4' long and MH
is normally described as best when lighting a single 2' square area so a 90gal tank would require 2 MH
lamps. One about 1' in from the left and 1' in from the right. If your 90 gal
tank has a center brace and the bulb is over the brace most of your intense light is going on that brace vs the tank and thus a horrific shadow is cast into the tank aswell. Having two bulbs on either side of the brace will cancel out the shadow cast by the brace.
Some 10K bulbs are more yellow than others so thats another potental for being visually different. Increasing the kelvin of the bulb will make the light output more white or blueish white.
As steve suggested the depth of a 90 is better suited for 250W but if your going to keep medium to low light corals at the bottom of your tank the 175's will work.
You felt the PC
's lit the tank beter because they emit light over the entire lenght of their bulb and thus you have a more even light albeit less intense over all. If you put a second MH
over the tank and position them as I suggest I suspect you will be much happyer. In addition you can swap out the bulb with a different brand 10K or even a higher K
rated bulb to change the visual color of the light. Below is a link showing you a nice compairson between bulbs. The compairson was done with 250W MH
but its still going to show how even bulbs in the same K
range can look very different over the exact same tank.