I'm not sure about this one with live sand. I know people replace their sand-beds but I would assume they do it with sand that has no bacteria in it. The reason I would hesitate to use live sand is because it is alive but also has a lot of dead bacteria in it too. I'm not sure if one bag would be enough to throw your tank into a cycle or not. Hopefully someone else will add to this.
First off make sure you rinse the new sand off to get all the junk out of it...
Next don't add it all at once. This will help reduce the chances of a cycle. You can reduce the cloudiness of the water by dumping the sand down a tube of some sort so it doesn't contact the flow of the tank before it gets to the bottom.
In reality, just don't go fast and you should be OK.
i just want to add another 20 pounds because it is not deep as i would like it i allready have 40 lbs so if i add like 5 lbs a week that should be good?
it is a 40 breeder
If it were me, considering the tank is running and stable, I wouldn't really hesitate to add 20lbs of live sand to my tank. Maybe do half today and the other tomorrow.
I would add it all at once.. its established already .. just spread it evenly to not cause any die off of the fauna in the current sand bed. A thinner even layer will allow the pods etc a chance to re adjust tot he sand bed and move up to inhabit their normal space in the new sand.
Slowly I've tried both ways in the past and when I added a bag over 2 weeks I saw no swings in anything. The tube idea is great a piece of left over PVC would work good just make sure it is rinsed real well the tank is still young and adding it at once could cause instability. Good luck.