Jim Morcombe wrote:
A couple of threads have made comments about cycling in a new tank that
I disagree with. For example, that it is impossible to cycle a new tank
without harming the fish and hence you must use "rubbish fish".

I have never lost a fish in cycling in a tank. Probably the main reason
is that I really understock the tank to start off with. Here's my method.

Most of the time I start a new tank is when my fish have just produced
their eggs so I have a couple of weeks notice in order to get ready. In
this case, I put an extra filter into one of my tanks and let it run
there so that is is full of bacteria. Sometimes I don't have this luxury.

I keep my fry in a net in the main tank for a couple of weeks. When I
am ready to give them their own tank, I take the dirty filter medium out
of one of the canister filters and rinse it out in the new tank. The
water turns into a murky grey/green soup. I then put the filter in the
tank and let it run for a couple of hours until the water is a little
clearer. I then dump the juvenilles into the new tank.

The "pond scum" from the canister filter settles all over the bottom of
the tank, making it pretty gross for the first couple of weeks. A lot
gets sucked into the filter, but far from all of it.

Thats it, the tank is now established. After a few water changes, most
of the pond scum has disappeared, but by then the bacteria is well
established in the tank.

With this method, I am putting in much more bacteria in the tank than is
needed for the tank, so the bacteria level will fall as the tank
establishes itself rather than building up.

When I set up a new tank at school for my science students, I often
don't get around to establishing the new filter first, but this doesn't
seem to make much difference. I make a party trick out of making the
water as murky as possible and then dumping a few fish in straight away.
The kids always accuse me of trying to kill the fish and are amazed
the next day to see the fish swimming around happily in much clearer
water. I then use this as a lead in to explaining the nitrogen cycle.

Note that although the bacteria level drops off, the algae in the pond
scum does not, so this does not add to the level of ammonia in the tank.

I have to wonder if you have ever checked the gill tissue of the fish
you have used to cycle a tank (without using filter media or gravel
from an established tank). Even with doing many water changes the
cycling fish still become "harmed". Their gills show burn damage.
This isn't my opinion, it's a fact.
So just because they aren't dead doesn't mean they aren't harmed. They
can also live many years and you would never know that their gill
tissue is scarred up.
Also, as for simply squeezing an established filter's media into the
tank and leaving the muck behind and *not* the actual filter pad, you
have not added the nitrifying bacteria to the new tank. The bacteria
secrete a glue like substance and adhere themselves to the surface of
the pad, gravel, tank walls, plants, decor, etc. of the established
tank.
They do not fall off the filter pad when you squeeze it out, nor do
they float about in the water as some people may think.
This sticky substance was found by scientists within like the last 10
yrs.
Many older hobbyists still think you can take the debris or squeeze out
a filter pad and seed a new tank, however, you cannot.