Upgrading PC with MH

[LBCBJ]

Alright, I have an 18 Gallon Aqua Via with 36 watt x 2 PC's (which I've heard are crap from this brand). Anyway, I'm planning on keeping probably mostly softies, but I definitly want clams, which I prefer even over corals. I know the PC's aren't enough, but I've seen pics on the net of people who have taken out a PC in their hood (and left the other one) and replaced it with a MH (the same PC hood I have) but none of them will reply to my post. So, my question is does anyone know how to do this and if so, where I could buy the parts. (And does it work over a long period of time). Thanks.

 

[LBCBJ]

By the way, my aquarium is 20" tall, so I was wondering what wattage MH I should go with
(150, 175, 250)?

 

[Ti]

go 250W,

 

[LBCBJ]

So you don't think 250 plus 36 PC is too much for a 18 gallon? I know clams like a lot of light.