04-03-2010, 07:57 PM
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A "cycled" tank is one that has gone through the initial nitrification bacteria process and there is now a colony of nitrosomonas and nitrospira bacteria living on surfaces in the aquarium sufficient to handle the amount of ammonia (and then resulting nitrite) entering the tank from the fish and biological processes (organic waste breakup, etc) or the "ammonia" used to effect the cycle. At this point it is safe to add a few fish, they will increase the ammonia and the bacteria will multiply to catch up. Provided the tank is not overstocked, or new fish are not added too many at a time (leaving a few days between new fish), the cycle will continue.
"Established" means the above has obviously occurred, but beyond that the biological balance in the tank has basically been set. Each aquarium will have its own balance, what we term the biological equilibrium, which means that all living organisms in the aquarium are basically in balance with each other. The fish load is balanced with the bacteria and the plants. This usually takes a few weeks after cycling. The biological balance can also be upset, just as the nitrification cycle can be upset, and by the same means. Doing something that increases the fish load out of proportion to the plants and bacteria, such as killing off fish, plants or bacteria in a significant amount (as opposed to one fish dying and being promptly removed), adding too many fish for the bacteria and plants to handle biologically, etc.
A sign of an established aquarium is stability. Stability in water parameters--especially the pH of a tank which can fluctuate during the first 3-4 months before the biology is established. This is easier in planted tanks because the plants do an incredible job of biological balancing by removing ammonia, toxic metals, all sorts of "stuff" produced by fish, and using organic matter that has been broken down by bacteria in the substrate. Partial water changes achieve much the same result, provided the parameters of the tap water are relatively similar to those in the aquarium; nitrates are (or should be, provided the fish load is not beyond the capability of the system) kept under control with regular partial water changes. In a well-planted aquarium, nitrates are often zero or very low, less than 10 ppm; this is a good indication of just how effective plants are at "balancing" the biological system. Without them, nitrates usually rise to very high levels, and as mentioned the regular partial water change is the best method of keeping them within reasonable limits. This is one reason why partial water changes can be fewer, and in some specific systems, unnecessary in well-planted tanks.
The type of fish as well as their number determine certain aspects of the biological balance. As do plants. And other organic-based items like wood; and also rock which in some cases may affect the hardness and pH, which is fine if that is needed--and this can be another means of establishing a more stable environment. Left alone, the pH will gradually fall, but buffers like rocks, or partial water changes can maintain it in a more stable manner.
Hope this has answered your question.
Last edited by Byron; 04-03-2010 at 08:06 PM..