That is only one, but they are so important that it should count as a few. Plants are the backbone of your water garden. If you have enough and the right type you really don't need much else. It will never stop amazing me at the level of maintenance that disappears just by having enough filtration and shade plants. Our courtyard pond gets a single dose of Aqua-One in the spring and then NOTHING else! It is roughly 15 by 8 with a Savio skimmer, endless cascades waterfall box, and a planting pocket.
There are anywhere between 4 to 7 lilies in the pond during the year and a lotus. There are hyacinth in the stream and the cascades is packed full of filtration plants. Also, not to be ignored are many plants in the planting pocket. If you haven't seen one of these, its basically a bog filtration pocket on the side of the pond. There is no water being forced through the pocket, but the underlay sides allows water in and out, but keeps the dirt in the pocket. A great visual buffer zone between land and pond also. You can pack this full of filtration plants and tropicals. They are largely under-valued. This is the only picture I can find of the pocket, but I know I have better ones at the store, I'll update shortly.
I'm not saying that you will never need to treat your pond again with enough water plants. Early in the year they are not established enough to starve out the algae, so early treatment is necessary, or a UV. But when you get the right balance, it minimizes the maintenance like nothing short of an ultra violet light.
