as above. The nutrient levels are most likely too high in your tank. The easiest way to deal with that is to introduce less nutrients (feed less) and to remove as much wasted nutrients as possible. Take that turkey baster or a powerhead and blast the rocks before your water change. Stir up the top layer of sand. If you have live rock with nothing on it, shake it around int the tank or a bucket of SW to remove detritus. Bottom line, get as much crap suspended in the water before doing your water change.
When you add FW, only use RO... preferably RO/DI. Our tap water is way too crappy to just throw into a tank.
Also, if you are feeding frozen, which I recommend, thaw the food in RO water before feeding. I thaw mine in water and then use a kitchen strainer (fish nets work well too). That water is loaded with nitrates you don't want in your tank.
HTH