I agree...that appears to be a very stable/mature & well maintained system. Nice selection of corals...do you have any issues with the tangs?
I know what some folks say about tangs in a 55 but I've got to confess...I do have a yellow in my 58 fish only(18 months) & a blue in my reef(medium size and just added after a 8 week quarintene)...you may have eventual size issues but everything sure looks happy&healthy.
Thanks for the eye-candy....