Voles and Moles are a large problem for most people and can cause alot of damage to your yard, even worst is the possibility of popping a hole into your pond liner. Their are a few different ways to help insure they won't reach your pond.
If you haven't installed your pond yet then look for a used carpet this works great for a under-layment. This helps keep them out because they are gonna dig where their is less resistance to them. Also you can use a regular underlayment, check out this thick underlayment.
If you have already dug out the pond or built it then you could bury screen or fencing into the ground around the pond so that they can't dig through it. This is what I hear works best it cost a little bit but beats trying to patch a pond. Using a PVC pond liner is the most effect for the money and quality.
If you are having problems with algae check out some of my other articles on algae removal. Getting Rid of Algae