I really doubt nitrates at 20 will cause any problems. Unless your tank sitter overfeeds, things will be fine until you get back. I think feeding lightly every 2 days would be in order so your tank sitter doesn't run up the nitrates.
I've seen ntrates up to almost 50 in my daughters tank and nothing died, just a bit of color loss in the corals and they went back to normal as the trates went down....
Go have a good time!
