If you are intending to have it as a nature pond,then you no way should you have paving slabs round it.It needs to have a gentle slope out of it for amphibians and any unfortunate mammal such as a hedgehog that might fall in ,to enable it to climb out.Slabs all round will prevent anything getting out.They will also be rough,insecure and look very harsh for something you are trying to make natural.
Take the slabs up and grass the edge or better still plant some nice plants that can grow over the side and look natural.Have you put soil in the bottom?you will need this for the plants,there are many you can have that will be suitable.Just look in any aquatic centre and take note of the depth on the labels that will match your pond depth.You need marginals,deeper ones and plenty of oxygenating weed.If you have a friend with a pond,scrounge a bucket or two of her water to get started with some insect life.Then stand back and wait for the creatures to arrive,which they sure will!!