I've had issues like that as well. Make sure you clean your equipment every once in a while. I know that you have a small tank and some people don't think protein skimmers are needed but I will say this: protein skimmers reduce organic waste and thus reduce nitrates. If they work on larger tanks then they most definately would on a smaller one. They arent required to keep saltwater fish and I'm not saying it will remove all your nitrates but it definitely can't hurt. With that being said, if you did decide to go that route, make sure the skimmer can more than handle your tank. In other words buy the best one you can afford.
On your next water change, take your power head and spray your live rock to get any old food or fish waste off. Siphon the sand really well. If you have a deep bed siphon small amounts each water change until you've done the entire tank then start over. I would change 1/3 of my water if they get to high. Make sure you rinse out sponge/pads with RO
water each water change...I would use clean saltwater if you don't have live rock so not to kill off your good bacteria. I have live rock so I change mine completely as its not my main source of bacteria for my tank.
You can buy something to chemically absorb them which is a smart move although finding out "why" you have them would be more beneficial in the long run. Just an opinion. =)