20 gallon NITRATE SPIKE HELP!!!

Hi I currently have a 20 gallon long fowlr but have upgraded all of my equipment to be suitable for a reef tank. This tank is 3 years old and has about 18 pounds of live rock and 25 of live sand. All parameters have been stable until I lost a serpent starfish about a month ago. I immediately removed his body before it could decompose and the nitrates remained around the 5.0 range. Well I ordered me some corals which are coming in within the next week and now MY NITRATES ARE AROUND 60 PPM!!!!!. They were fine a few days before I ordered them. how the heck this happened I have no idea. I have used RO water since I have had this setup. Only have 3 fish in the tank and a few snails and crabs. I have performed about a 40% water change yesterday and a 20% earlier today with what water I had prepared. Its not doing much good at all. I am in desperate need of some quick suggestions. No I do not have a skimmer since I do weekly 10-20% water changes without slacking. ANY HELP WOULD BE APPRECIATED THANKS!!!!!