Hi. There's a big problem with algae in a shallow pond. And if you want it chemical free, you need a really good bubble pump and filter system to run at night. Clean regularly to remove any leaves, etc. that have fallen to the bottom.
How shallow, if it is not too shallow, a large Plecostomus is an algae eater and will help get rid of algae along with good water circulation. If it is too shallow, the water may get too warm to support the fish. If it is algae free, a few algae wafers a day will feed them without a problem. They can get to 18 to 24 inches long, so they are not small. They will need some cover to get out of the hot sun, some plants or rock overhangs or something. I have them in my aquariums and never have a problem with algae. When we had a privately owned pet store in town, they dealt a lot in ponds and their fish and plants, they always sold them plecos to help keep the algae at bay. He had a program where we could bring in our plecos as they grew out of the size of the tank and get a baby free and he then used the large ones for people with ponds so that they would start with large ones. I used to turn mine in at 18-20 inches I have large fresh water fish tanks).
I've had several water features at different properties we've owned. If your shallow reflecting pool gets a considerable amount of sun it will grow algae very easily. You will need to put in additive to prevent that. Your filter will need regular cleaning as well.