Frog I found

Its a bit hard to see this little frog. It is about half the size of a matchbox. I have a bathtub in the ground outside for a couple of local wild ducks that visit me. I went out to top up the water and found a little frog stuck in the water, he could not get out. Now he is safe in a tank I made a home for him in. He is not a protected species so I may keep him yet. I don't know how he got in my backyard.