In my opinion, you don't need a drain in the pond floor. If you think you might possibly want a way to do very small partial water change at some point, I would suggest a drain from the bottom of your waterfall, rather than one in the floor of the pond. I have a long pipe in the bottom of the waterfall filter receptacle. The pipe/hose is buried and comes out into my garden, a ways away. There is a gate valve in it. (Pull the handle up, let some water out. Push it down, close it off again.). When I'm doing pond housekeeping, I turn off the pump. As I'm puttering around, the water in the waterfall filter has time to settle. Gunk and crud go down to the bottom. Before I turn the pump on again, I open the gate valve and a bunch of nasty, funky water comes out into the garden, which the plants love. Depending on how I feel at the time, I just let that first bunch of water out. Or, I might let the waterfall receptacle empty completely. This is not a drastic water change, by any means. When I turn the pump back on, I barely notice any difference to the water level. Another nice thing about having this kind of set up is that when we have torrential rains and the pond is really, really full - it's an easy way to let out some excess water. I do this with the pump on. For me, this is all super efficient and a simple way to do the kinds of things I need to do with my pond. It's just a garden pond with a few goldfish - nothing fancy. If you are just in the planning stage, maybe look into this to see if it's something you might be interested in?