09-14-2012, 08:43 AM
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I usually do 5% Water Changes once every month or maybe every other week (depending on the size of the tank) just to replace trace minerals. Water changes in marine aquariums do not have the same effect as in freshwater aquariums.
In freshwater aquaria, Ammonia is excreted by the fish in the form of respiration, urine and feces. This ammonia is broken down by oxidizing bacteria into NitrItes. These nitrites are still harmful to fish, but Nitrobacter bacteria break it down into NitrAtes, which are not as harmful to fish in moderate levels. Through regular water changes we reduce the nitrates to manageable levels in the aquarium, but never near zero ppm. That would take a 100% water change every week, and that would be stressful the fish. I do a 25% water change (5 gallons) in my twenty fresh on the first Saturday every month that begins with a two (hence this month it will be the 22nd).
In Marine aquaria, we are trying to achieve ZERO ppm on Nitrates. So when the nitrogen cycle turns NitrIte into NitrAte, the cycle cannot end there. Nitrification reduces the buffers in an aquarium, lowering Alkalinity. So Nitrates have an adverse effect on water quality, making it hard to keep Calcium and Alk in check. Since water changes alone only change the percentage of Nitrates in the system (never reduce them to zero without a 100% stressful wc), we need to attack the problem beforehand.
Your live rock has deep-seeded denitrifying anaerobic bacteria that breaks down nitrates into Nitrogen gas that leaves the system naturally. A ddep sand bed (4"+) also harbors this denitrifying bacteria that breaks down nitrates into nitrogen gas. These two things coupled with a protein skimmer (which removes Dissolved Organic Compounds, eg urine and feces, before they are broken down by the nitrogen cycle. Activated Carbon also removes DOCs, but is a controversial method) can reduce and maintain a reading of 0ppm nitrates (or at least to a reading that is less than 5ppm).