I am plumbing a pond and have some above-ground PVC pipe. I used the purple and blue glue to attach the pipes, but am concerned some pipes could still come loose. I plan to wrap the pipes with duct tape, but wonder if there is a better solution, as duct tape doesn't do well in situations involving lots of water. How can I reinforce the pipes to make sure they can't fall apart?
Either you glued them correctly (I prefer "cleaner, then primer, then glue" for the most reliable connections) or you'll need to re-glue them after the joints fail.
Nothing you add to the outside will change that.
You do need to paint or otherwise cover them to prevent damage from the UV in sunlight, since you mention that they are above-ground.
Duct tape is a poor choice, it falls apart outside rather quickly.
It won't fall apart. It just won't. PVC glue doesn't work like most glue. It's not a sticky substance that goes between two surfaces and holds them together. Instead, it chemically melts the pieces together, basically forming one piece. PVC is used all the time, in homes, ponds, pools, factories, basically everywhere where pipe is needed. You don't see them wrapping the connections with duct tape, do you? They are gluing it together just the same way as you. So don't worry about it, I promise it won't come apart any time soon.