I would like to clear up a misconception regarding cycling and bacterial colonies that many fishkeepers have.
The bacterial colonies in any tank always size- either up or down- in response to the available food supply. This is essentially ammonia. The bacterial colonies will always be most dense where the confluence of food and oxygen is greatest. This is normally in the filter. However, they colonize all hard surfaces of the tank. Where they basically are not is free floating in the water. Adding water from a cycled tank to a new tank does virtually nothing to aid the cycling process.
This means that when there is an increase in ammonia in a tank, the size of the bacterial colonies will grow accordingly. Conversely, if you decrease the the available ammonia, they colonies will shrink in size. Put simply- add a fish and the colony will grow, remove a fish and the colony will shrink.
So what happens when you remove media and/or gravel, decor etc from a fully cycled tank to help start up a new tank? If you put a new filter onto a tank, this does absolutely nothing to increase the size of the bio colonies, what it does is spread out the existing bacteia. Since the filter is normally the optimal place for the bacteria to live, there will be a spreading out of the bacteria, not and increase in the total colony size.
When you move that newly colonized filter to a new tank two things occur. First you introduce a colony to the new tank but you also reduce the colony size in the exisiting tank. Move too much and the old tank can spike. The same applies when you move gravel or decor from an fully cycled tank to a new, uncycled, tank.
One key to understanding what is going on with cycling is to realize:
In comparison to other types of bacteria, nitrifying bacteria grow slowly. Under optimal conditions, it takes fully 15 hours for a colony to double in size!
Since few tanks present "optimal" condition, you can figure it will take longer.
The opposite side of this coin is when you remove food source for the bacteria, by decreasing the fish load, the bacterial colonies will downsize to get back into balance with the reduced food supply. So if you remove fish from a tank for a day or two and then return the exact same fish, the tank will no longer have an adequate colony size to support this load and there will be a spike of some amount.
As for using a pleco to cycle a tank, I would never do it. If you need to have fish in your new tank while you are away, I would urge you to buy some hardy cycling fish like z danios or barbs and use them rather than a prized pleco. Even if the pleco survives the process, the odds are good it will never be as healthy as it was before the process and it may even die.
One more note, live plants eat ammonia. So it you plant a tank you basically reduce the available ammonia supply by what the plants take up. What this means for cycling is that you need a smaller bacterial colony to have the tank effectively become fully cycled. For example, if the plant load is able to consume 50% of the ammonia being produced in a tank, then the size of the bacterial colonies need will also be halved. This also means that the overall cycling process will be faster since the resultant amount of bacteria needed to process wastes is smaller and thus takes less time to grow to the needed size.
. when i set up the tank im going to use a quarter of the water and some gravel from my existing tank. taking this into account plus the fact that it will have roughly ten days to cycle anyway , will my common plec < ronan > be able to help it cycle while im away if i put him in the daay before i go ( someones coming to feed them twice a day , and im planning to keep him in this tank )
< and i wuv him too much to give him to the scummy lps who said they would take him , they asked me what a plecostomus was! >. he has bulgier eyes than other commons? anyway i wont cycle with him then
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