Question

[stanleytli]

My tank is cycled, after 20h ammonia and nitrite = 0
I am expecting new fish in about 2 weeks, what I suppouse to do now with my tank?

 

[TwoTankAmin]

Every 2 to 3 days add ammonia equal to about 1/2 the amount of the full dose you used to cycle the tank. The bacteria do not need to be fed daily, they will be just fine. However, be sure when you are ready to add fish that you do a big water change and get the tank to the desired temp.
 
Remember, your tank is cycled for full average stocking. If you put in only a few fish, you can no longer add ammonia. Over time, the bacterial colony will shrink back to equalize with the bio-load of a tank. It is a shame to make the effort to get a tank cycled for full stocking and not to take advantage of that. If one knows at the outset they wont stock well fairly soon after completing a  fishless cycle, the whole process can be made to accommodate that.
 
One merely does the fishless cycle using less ammonia. Instead of a 3 ppm level one can use a lower amount. You can cycle a tank to process 1.5 ppm of ammonia faster than you can for 3 ppm. Its simple math, it takes less bacteria which means they take less time to reach that level of bacteria.

 

[stanleytli]

Thanks @TTA
My tank 500l is for about 10-12 discus, so strong biology filter is very important.
I was hoping to get new fish last weekend, but unfortunatelly I will get my discus in about 10-14 days.
I will be feeding bacteria as you suggested and one day before stocking I will do a big water change and heat up to 30 deg.
Water for discus has to be very soft and with ph below 7, but I hope it will not affect bacteria, as they were growing in water harder with higher ph.

 

[TwoTankAmin]

How much below 7.0? If its much, you will likely need several more weeks to get the tank cycled at those levels.. As soon as you go to work on dropping the pH you risk stalling or even killing the bacteria depending on how you try to do it and how low you want to go
.
Unless you are going to be keeping wild discus, there is no need for water below 7 and how soft is not as big a deal either. There is soft and then there is soft. When I kept discus I had them about 7.4 pH, GH 6 dg and KH 5. That was TDS about 105 ppm. They spawned too.
 
The bacteria want ammonia as NH3 and the lower the pH goes, the less of this there is. In a tank with a pH of 6.5 and a temperature of 86 F and tested at 2 ppm of total ammonia, the amount of NH4 in that 2 ppm will be 1.9946 and the NH3 .0054 ppm. Now take this all down the the undetectable levels in a cycled tank and there will basically be 0 NH3 ammonia the bacteria will take in.