You should follow below rules when setting up the substrate:
The root are the best if for a long time remained under water (in pure natural water reservoirs) and if it drown in water. If the wood is not sinking you can cook it for about an hour in a water with salt. When this treatment does not work then there is one more think you can do - attach the root to the bottom of the aquarium by attaching it to heavy stone etc.
When choosing decorative materials you have to make sure that decoration is free from calcium and metals which can have a detrimental effect on fish and plants. We have to be careful not to overdo the decorating and leave space for the fish to swim freely.
To make aquarium decorating task as easy as possible you can create a diagram of the aquarium bottom and it's elements like roots, stones, plants etc. Below is an example of a plan for an aquarium with a capacity of about 160 litres
The substrate is made of medium coarse gravel. It can also be mixed with the fertilizer.