Yep, 400w or something like that will be algae city on a tank like that. With larger tanks, it's harder to maintain adequate co2 concentrations, mostly due to poor circulation and huge surface area. I'm seen people start out with 6x80w over a 180w tank and end up with a tank full of algae.
(Edit: Also, I second Tom's suggestion on the 36" 39w T5 fixtures. Most "72 inch" setups use 80w T5, with 80w T5 are only 5 ft long, so you don't get the spread over the entire tank, better to use 2 of the 36" bulbs end to end to get a full spread)
As I mentioned earlier, I'm planning 3x65w PC and 3x175w MH, with the space to add 3 more 65w PC later if I want/need. Even with the start up lighting, it'll all be staggered with short photoperiods in the beginning. Even if you install a 6x80w T5 setup, I'd start with only 2x80w for the first 3-4 weeks, then bump up to 4x80w, and then maybe after 3 months use the full 6x80w.
As Tom alluded to, maintenance on larger tanks is of course harder, so it's much more enjoyable if your plants aren't growing 1"+ a week, so less light on bigger tanks is more enjoyable.
(Edit: Also, I second Tom's suggestion on the 36" 39w T5 fixtures. Most "72 inch" setups use 80w T5, with 80w T5 are only 5 ft long, so you don't get the spread over the entire tank, better to use 2 of the 36" bulbs end to end to get a full spread)
As I mentioned earlier, I'm planning 3x65w PC and 3x175w MH, with the space to add 3 more 65w PC later if I want/need. Even with the start up lighting, it'll all be staggered with short photoperiods in the beginning. Even if you install a 6x80w T5 setup, I'd start with only 2x80w for the first 3-4 weeks, then bump up to 4x80w, and then maybe after 3 months use the full 6x80w.
As Tom alluded to, maintenance on larger tanks is of course harder, so it's much more enjoyable if your plants aren't growing 1"+ a week, so less light on bigger tanks is more enjoyable.
