Cycling A New Tank Using Established One?

The best way is by using some filter media from the 40 litre tank. The substrate will have some bacteria but not as much as the filter media and to use it for maximum effect you'd still need to put it inside the filter.

It is safe to take up to one third of the media from the 40 litre tank. Refill the space left with media from the new tank. Then put the used media into the new filter, cutting it up if necessary to make it fit, add ammonia and follow this method for fishless cycling. Although the bacteria won't die off if you don't add ammonia for a day or two, it is better to transfer the media immediately before adding the first 3ppms-worth of ammonia.

Keep an eye on the ammonia and nitrite levels in the 40 litre tank for several days until you are sure you there is no mini spike in either of them. If you do see a spike, it won't last long. Just do a water change as needed to keep them below 0.25ppm.




If you don't want to do a fishless cycle, it should be safe to add fish equal to around a quarter of those in the 40 litre. The reason for adding slightly less fish than the proportion of media is to allow for slight die off of bacteria during the transfer. Then add fish slowly, a few at a time and monitor the ammonia and nitrite levels (doing water changes if necessary) and only adding another batch when you've had double zeros for at least a week.

If you are going to use fish, make sure you keep an eye on the water condition. Zebra danios are a good choice, they will like the bigger tank with them being such fast swimmers. If you move just the 3 fish, once you know the filter can cope with them get more starting with just one fish  - a third of what will be already there. When you have four fish, the next time you can add two. Try not to add more than a third of the fish already there in one go. Yes, I know that adding 2 fish when you have 4 is more than a third    but you should be OK in a tank that size.