Whatever wood it is (and I think it could be beech), it's not nearly old enough for an aquarium – nothing with bark still on it will be!
If you want to go down the 'found wood' route, go for a very old, heart-wood branch from under an oak tree, or a very old branch that has mostly rotted away under water for several years. Wire-brush it well to get rid of any soft material on the surface and when you are down to solid, hard wood, it should be safe to use...