Tank sitter DEFINATELY! Try to get someone knowlegable. I ask the kids from my LFS. just make sure you put your valuables away.
The last year, we vaca'ed for 3 weeks, and asked the housesitter to feed, and moniter, but he didn't know to watch the temps (this was prechiller). It was really hot when we were gone and the tanks got too warm, which caused a viral/bacterial bloom, and 1 week later all 20 horses were dead.
Moral of the story, find a tank sitter that knows about tanks, or if you can trust your LFS, bring your horses in and ask them to BBsit. I've done this too, but I spend hours a week @ the shop and I know everyone there.
HTH