nitrate spike in 3rd week

Hi guys, I've started a SW tank 3 weeks ago. I top off with RO water every week when the water evaporates and drops to a certain level. 1st Week - Let the tank cycle, there's 1 three stripe damsel in it since i started. 2nd Week - Nitrate at 0.3, threw in 2 small cubes of frozen brine shrimp...