Birch is a tree which, in summer, contains a lof of glucose in its veins - therefore if used fresh, it will contaminatte your water. However, it is spring, and if you cut those branches before the bulbs start growing, and put them into a vase (allowing them to use the glucose in the branches) you might stil be able to use them.
Further, the wood is very soft indeed - thus very vulnerable for decay. that does not make the wood unfit in all circumstances, but personally, I think you should only use it - a little - in case you have wood eating plecos in the tank (plural is intended)
but otherwise? I think you better not use it