You will find lots of pics of my pond in the coldwater section. Have a look at the entire Photobucket pictorial of the build from scratch here
I have had my pond 5 years and have yet to clean it at all.
A lack of plants will not be helping. The green gunge build-up on the bottom is natural. I happily leave mine there although I am aware it will need to be thinned out eventually :-?
If you have a lot of overhanging plants or trees these will be dropping leaves into the pond, adding to the gunge. Trim them back or cover the pond.
If you plant a lot of plants to cover no more than 3/4 of the surface, it should help keep the water clear.
Too much direct sunlight will make the water go green. Unsightly but good for the fish. The white ones especially sa it protects them from cancer.
If the pond is not too big, or an awkward shape, you could put shade cloth over it. Either up high like a shade sail, or shade netting down low just above the water line.
My pond is full of oxygen weed which I pulled from the river that runs through the middle of town - free. I also have a number of lilies.
I will go and take more pics of the pond as it is now. At some times it goes green then it goes crystal clear again.