Join Date: Sep 2005
Location: Jakarta - Indonesia
Safe those troubles related to too much light. I have two 260g tank both with 3 x 150W MH. I found them already quite strong and it does penetrate real good to the bottom of the tank that some sensitive plant will die due to too much light, plants near or above the surface directly below the light might melt/dry up. That is why I raised the lamp a bit higher, and you may find that you need to keep it at least 12" from water surface.
In fact I keep my almost ridiculous 400W MH lamp at almost 3 feet away from water surface so that it works as it should, not blistering my plants.
I would opt for mild 70W, 150W at most, double ended MH and combine it with the existing PCs. 2 of them I think will be enough to cover the tank area.
Blast the MH for a shorter period (30-40%) if you need it and let the PC handle the rest. Look for those with 6500-10,000K range rating. Above that is usually very blue and not really much usable in spectrum. Below that you might find it too yellow even though they work great.