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hey all i have a 20 g tank i had for two years i know its dum but i just set up another 20 this sunday that just passed was wondering when to make my 1st water change cuzz i forgot its been so long since i set up a new tank.

has your new 20 cycled yet?

what are your water parameters?
i usually hold off water changes until the tank has fully cycled.
are you fishless cycling or have you already added fish to this tank?

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Truth doesn't always win friends but it influences them

i have 6 danios and 1 darf and 1 swordtail for cycle and i use stress zyme etc to make cycle faster

wait till the tank is cycled to change water, changing water too soon will only lengthen the process. when yout nitrites are gone and nitrates are coming up, it'll be time.

If you wait until the cycle is over to do a waterchange, the concentrations of ammonia and nitrite in the middle could become deadly.

If you have a test kit, you should never let the concentrations get over 1 ppm if at all possible. So long as you match the temp, pH, and hardness of your water with the tank, you can do as large a water change as possible. This is good so that you can keep the concnetrations as low as possible. I usually recommend small but very frequent water changes (like 10% a day) and larger when necessary, until both the ammonia and nitrite readings are consistently zero.

You will not lengthen the process because the bacteria colony will continue to grow so long as the rate of ammonia production by the fish is larger than the rate of ammonia consumption by the bacteria. The rate has nothing to do with the concentration that is in the water at any moment, that is, when the tank is cycled, the ammonia is zero, but the rate of production from the fish is the same as the rate of consumption by the bacteria both of which are definitely bigger than zero. Because you are trying to match rates, you can keep the concntrations low to be much safer for your fish and keep the cycling process right on track

I wrote a quick piece on fishless cycling some time ago in whcih I described my chosen method. Which was to add fish food in small quantities daily as if there were fishes already present.

The rationale is as follows. The bacteria colony takes time to build up, even though bacteria reproduce at a fair rate of knots. So, I provide a steady input of food in measured quantities, so that the bacteria aren't overloaded. Some people make the mistake of dumping a large quantity of matter in there and letting it rot - which will eventually work, but create complications along the way.

The comparison I made was as follows - dumping a large amount of matter and letting it rot is akin to someone dumping a dead buffalo in your living room - pretty soon that living room is going to be a fetid, stinking hovel you won't want to approach within 200 feet of! Adding measured quantities of food, on the other hand, is akin to sticking bacon rashers in the fridge on a regular basis - you get to wake up to a bacon breakfast each morning. Give the filter bed bacteria a 'bacon breakfast', as it were, and they'll establish themselves with fewer complications. You can increase the food dosage carefully to reflect your intended fish loading during the cycling process as a refinement.

The point made above by Bignose about setting up a dynamic equilibrium is the important one though. A water test conducted with a test kit is, if you like, a 'freeze frame' snapshot of your water conditions. Initially, during the establishment of the filter bed, ammonia production by your chosen source(s) will exceed the rate of ammonia removal by the bacteria (ditto for nitrites further down the cycle), and so a test kit will show a non-zero reading. Once the bed is established, the rate of production of ammonia will be matched by the rate of removal by the bacteria, and the 'freeze frame' snapshot of the water chemistry dynamics taken with your test kit will show a net zero reading. This isn't to say that there isn't ammonia in there at all - after all, your fishes are excreting it! If you were able to follow one of your fishes around with an eye dropper and take water samples immediately from the fish's urogenital opening, the resulting samples would test positive for ammonia. However, in a dynamically balanced setup with an established filter bed, that ammonia is carried away by the filter currents, into the filter media, and metabolised by the bacteria. Consequently a sample taken from close to the water surface, which consists of water that has passed through the filter bed, will show a net zero reading because the ammonia has been metabolised.

An aquarium is, basically, a whole set of such dynamic equilibria that you, the aquarist, are responsible for managing to some extent, be it either via high technology (big external canister filters, activated carbon, ion exchange resins) or biological filter beds. Some of course use both approaches.


Panda Catfish fan and keeper/breeder since Christmas 2002

So to summarize:

Since the tank has nice fish in it, worry more about the tank than the cycle. Do small water changes and test for ammonia and nitrite, every couple of days.. Feed very lightly for the first month. (don't let their tummies stick in). The filter bacteria will grow no matter what you do but the fish may suffer depending on what you do.