Do you have any knowledge about saltwater?
Would it be possible to remove the live rock most of the pests are on and then kill the rest? Did you say 500gal? Oh to only have your problems...sigh
Sounds to me like you are gonna be getting your arms wet.
WOW! just went back and looked at the pics! You would be squirting joes or vinegar for a very long time.
How much time effort and money you want to put into this? That might have a bearing on how you want to tackle this. Looks to me like there is enough good stuff in that tank to make a tremendous looking reef someday but its gonna take a lot of effort.