Oak will work in an aquarium.
It leaches a lot of tannins.
The finer the branches the faster it rots (any wood).
Manzanita also has fine twigs, if you like that.
The problem with using unknown wood is the possible toxins. The best ways to deal with that are 2- age the wood, alternately soaking and drying, or boiling it.
Either of these are not great things to do to fine twig-sized material. It starts to fall apart. Start with a wood that is known to be non-toxic.