Ok, [rant mode on] I have seen a lot of people repeating the lie that deeper tanks need more light. That might be true if you are using crappy lighting systems or barely have enough light as it is. But if you use decent lighting systems with decent reflectors the depth of the tank doesn't mean squat. You might hear about spectral drop off or lighting decrease. If your tank was 30 feet deep it would be one thing. But the difference between 18" and 30" is not enough to worry about as long as you have good reflectors.