To keep the turtles in I'm basically using a vertical Wall with a decent gap from the waterline to the top edge of the pond. Experiments with my best climbing turtle has so far seen it not make a 10cm jump up, but I'll either double this to be sure else tile the waterline so there's limited grip. Until I'm certain they can't escape, I'll fence the outside perimeter of the garden bed.

To keep the toads out is also within the vertical wall design. Toads are selective breeders and prefer still water with shallow banks & limited surrounding vegetation- more like a dam or even road side drain. This pond will have thick surrounding vegetation & no banks, plus a 40cm external wall. I'm sure the odd high jumping large toad will get in, but they're too big for the turtles to eat & be poisoned. And should (heaven forbid) 2 get in and lay eggs, I'm confident the current of the filters will suck up any of the poisonous eggs before they could be eaten.




These progress photos are from my building blog, & more detail on the construction is shown there. http://aqua-blog-building.blogspot.com/search/label/Pond




The foundations are 30cm deep & well reinforced. Though the pond won't be deep it has a weak point at the bridge so we needed it to be able to function as a monolithic structure pretty much!




The fondations also held the upright reinforcing for the block work. This will support the vertical walls from the horizontal pressure of the weight of the water.




The blocks themselves are H blocks, designed to be core filled & also allow for horizontal reinforcing to fit within each row. So at this stage there is a continues band of reinforcing rod around the perimeter of the structure, + all the uprights. Once the concrete is in it will be very strong.