Excel will be costly on a 180 gallon tank, but it can be used well on such tanks.

Generally about 300-400w is all you;d need, you can use CO2 and have wonderful results in this range.

I have a 180 as well, and I have about 2/gal on Pc lighting on a pendent.
Then I also have several MH's as well I use for more light if I seek faster growth rates and plan on gardening more.

Most folks are better off with 2w/gal though and smaller tanks for Excel.

Regards,
Tom Barr