In established aquariums, just as in nature, toxic ammonia from fish waste is broken down by bacteria into nitrite, which is itself broken down by a different group of bacteria into nitrate. In a newly set up aquarium, those bacteria are not present in any quantity, and it takes time - about a 4 to 6 weeks under normal circumstances - for those bacteria to multiply to the point of being able to keep up with the waste output of the fish. “New Tank Syndrome” and “The Break-In Cycle” describe the period in which ammonia and then nitrite levels rise to dangerous quantities before being converted into relatively harmless nitrate.
A tank need not be “new” to go through this break-in process. That thirty year old metal frame tank you drag out of the attic will cycle just like the sparkling new acrylic one you just bought this afternoon. Even well established aquariums can become “new” again in terms of the break-in cycle. Removing large quantities of bacteria, by changing the aquarium gravel for instance, will cause an “old” tank to go through another cycle. Poisoning the bacteria with medicines or shocking them with chlorinated water or sudden temperature changes will also set the cycling process back to day one.
New tanks are generally broken in by adding a few hardy fish and simply waiting out that first month. It is important to note that the cycling process does not begin until fish are added. Some hobbyists set up their aquarium for weeks or even months before adding fish, and are surprised to see high ammonia and nitrite readings shortly after fish are finally added. Smaller barbs (tiger, gold, rosy), larger tetras (head and tail light, red eye, red minor, buenos aires), danios (zebra, leopard, pearl, gold) and rasboras (heteromorpha, scissortail, redtail, brilliant) are hardy enough to withstand the temporarily high ammonia and nitrite levels and inexpensive enough to replace if some perish during a particularly rough cycle. Damselfish (blue, yellowtail or striped) and mollies are their saltwater counterparts. Anywhere from two to five inches of fish may be used per ten gallons of water - any less and the tank may go through another, but lesser, cycle when more fish are added later; any more and the ammonia and nitrite levels may rise beyond what even the hardy fish can tolerate. A few hardy Corydoras catfish (green or albino) may also be added to reduce the risk of overfeeding. Algae eating fish are generally unnecessary until after the cycling process is completed.
Once the cycle is in progress, it’s best to not add other fish until it is complete. The starter fish have had a chance to become slowly accustomed to the increasing ammonia/nitrite levels; any new fish have to deal with the shock of being dumped into poor water conditions all at once. It’s also best to feed regularly, once or twice per day, but sparingly. Any uneaten food can worsen the already poor water quality.
Periodic testing of water chemistry will help to determine if a tank is on course during the break in period. (See the Typical Break In page that follows.) Break-in fish may show signs of distress, including rapid breathing, loss of appetite and skittishness during the peak times of the cycle. It is especially important to monitor the fish regularly during this time, watching for ich and other stress-related diseases. Partial water changes of up to 30% do not substantially affect the cycling time and dilute the harmful chemicals, at least temporarily. Cutting back on the feeding will also help keep levels in check.
The total cycling time for most aquariums at 78-80 degrees F. is about thirty days, although it may take quite a bit longer at lower temperatures. On rare occasions, some trare occasions, some tanks will mysteriously stay at the nitrite peak for weeks or even months. Adding a handful of gravel from an established tank will usually cause the nitrites in these tanks to crash to zero in a day or two.
As the cycling process nears an end, many hobbyists notice an improvement in the appetite and overall deportment of their fish. In addition, a small green algae bloom will often accompany the drop in nitrites and rise of nitrates. The cycle is over and the tank ready for additional fish when both the ammonia and nitrite are zero and the nitrate has begun to rise. This is an appropriate time for a first routine partial water change.
Copyright © 1996, 2005 James M. Kostich
All rights reserved.
For more information, see " A Typical Break-In Cycle".