Where do you plan to get your LED's from?
I like some of the deals from
LED Aquarium Lights | Reef Tank Lighting | LED Group Buy Mostly the heatsink itself.
RapidLED.com also has some good LED's to choose from.
Then If you want to go cheap, a kit from
AquaStyleOnline is a great option. These come from china and shipping is by weight so order a heatsink from ledgroupbuy or rapidled to get a better heatsink for your money.
Next, lets talk optics. 30" tall tank isn't very tall. The light falloff from LED is much less drastic than that of MH and the penetration is high. Especially if you go with high end cree 5w leds which I think rapidled carries. 3w are still typical and a good option. I went with 60 degree optics on my rig and will always feel these are the best option for any tank. 60 degree optics are wide enough of an angle that the light will blend together well. 30-45 degree optics are tight and can make it look like beams of light in your tank. 85+ degree optics are a bit on the wide end so you have to move them closer to the water to have the same footprint as 60 degree. This can create more shadow area. In a 30" tall tank with 18" depth, you can almost guarantee that 85 Degree would be lighting your floor. Go with the 60 degree.
Now comes the numbers. You can feel confident with 1x 3w LED per gallon. That's going to be a lot more than you are getting from those two Metal Halides. For the long haul, I prefer to go with about 1.3 LED per gallon since they are dimmable. This leaves you with more option of color control. This may really help if you want to set up advanced lighting schedules for sunrise and set.
And the colors... Go with what you can. LED have a tight spectrum unlike MH and t5. For instance, If you went with just 10000k cool whites and 455nm actinics, you would be crawling into that boat of people that insist on supplementing them with t5 for more growth. To maximise coral growth with LED, you need to build a light spectrum that is full range where you need it. I go with about 50/50 actinic and white. I like to toss in a few deep reds on the actinic strand to make the reds in fish illuminate. 4 reds would be enough on a 150g tank. Being spread out, you need to cover more ground so get 85 degree optics for these.
Whites are more complicated, This is where I add a bit of everything if I've got the option to go with quality LEDs. since it is a soup bowl of light, making your Whites/blues ration might be better at 60%/40%. On the white strand I would go with primarily a mix cool white and neutral white (about 70% of the strand) then fill the other 30% with a mix of warm white, cyan and green. Remember that red light does promote algae growth so you want to limit this and with multiple drivers, keep your warm whites where they can be fully controlled. For instance putting all your warms on one strand or if needed, mixing some of the cool and nuetral whites in with that strand so that if you want you can turn the warm strand down and the dominant cool/neutral strands up to lower the amount of red while maintaining large amounts of white.
