A couple more things to be aware of. First, make sure the applewood is completely dead and dry throughout. This means collecting it from the ground, never still attached to the tree as it will inevitably have sap in it then. And it needs to have been off the tree long enough to have thoroughly dried out; thee larger or thicker the wood, the longer it will take to dry. Cut a small bit off of an end and see if the wood is completely dry--it will likely snap if dry--or still moist or pliable; if it is not totally dry, do not put it in the tank as it will leech the sap and this can harm fish.
Applewood is a hard wood I believe, so it should be safe as far as that goes. But any liquids it may have come into contact with will have soaked into the wood. Be careful where you collect it; near industry, farms, pesticides, fertilizers, oil...all are serious.