Before we did our 25% water change this morning we had a nitrate reading of 20. After the change, the nitrate reading was still the same. Generally after a water change it will drop to 10. The only thing different that we did was add a carbon filter to take out the rest of the medication and my husband cleaned off a lot of brown algae from around the top inside of the tank and cleaned the sides of the tank. If any algae had gotten back into the water, would that have anything to do with the readings? The ammonia and nitrite levels are at 0.