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Keep the fish tank clean. Test your water when you come back.
You really don't need to worry about this too much. Before you go on vacation, do a water change within the week before you leave. Make sure that if you have someone coming over to feed your fish they know how much to feed, and won't pollute your water. Clean your tank again soon after you return. Hopefully, nothing went wrong while you were gone, but check for any spikes in ammonia, nitrite, or nitrate. You may have to perform large water changes until your water is back to normal.