Early spring is the perfect time to clean and perform maintenance on your biological filter. During winter the water is too cold for nitrifying bacteria to live and they go dormant. As water temperatures heat up nitrobacter and nitrosomonas, the two “good” bacteria begin to colonize. If you clean your filters with chlorinated water the chlorine can prohibit the growth of these two beneficial bacteria — especially nitrobacter which, of the two, is the slowest to grow.