Those measurments come to about 17.5 gallons but you can still use that tank! As soon as you get your rock your ready to go! I wouldnt worry about a skimmer on a tank this size...If it were me id prolly go ahead and put about 10g's of salt water in there along with your powerheads and sand for a little motivation lol... The cycle wont start just yet tho! try and get at least 1 good piece of live rock asap