why do discus needs a cone or triangular tile to breed?

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may i know why do discus needs a cone or triangular tile to breed? cant they just lay their eggs on leaves or gravels??

[Keith Perkins]

In planted tanks fish that pair up do sometimes lay on leaves. They'll also lay on the glass of the tank, a lift tube, driftwood, or just about anything else they can find if they're in the mood. Spawning cones are just generally found to be the most preferred angle and height for discus. They're also easy to clean between spawns.

[John_Nicholson]

The purpose of a breeding cone is to try and make the female lay the eggs in a nice neat pattern so that the male can better fertilize the eggs. Sometimes when they lay on the glass they scatter the eggs all over the place which reduces the number of fertile eggs that you receive.

-john

[Pardal]

You can also add to all of the above Bricks, and 2'' to 4" PVC . just about anything when in the mood. I agree the point is to make it easier for the male
to fertilize as many eggs as possible.

[Apistomaster]

I have never heard this theory that an artificial surface helps make the male's job of fertilizing the eggs easier. I have had wild Discus pairs choose to spawn on the underside of a large wood branch before and I thought that was odd. Spawning on wood branches is the normal mode for all Discus in the wild. It did offer them a more private and defensible position to guard their eggs than the more usual exposed spawning sites but I had never seen them do that before or since. A male Discus really serious about fertilizing the eggs usually does well no matter where and how the eggs are placed. It's all instinctual behavior pretty well worked after hundreds of thousands of years of evolution. Using artificial substrates are more for the convenience of the breeder than the fish as any relatively smooth surface will be used if no more suitable site is offered.
My wild Discus all favor the wood branches I provide to the clay spawning cones I offer them. I liked and plan to try the sand filled plastic spaghetti container idea I saw posted in the wild Discus section because spawning cones are expensive and I have only happened to have domestic Discus use them. I know wilds do use them; just saying mine never have.
This wild Blue did choose a piece of slate but ignored a cone.
http://i157.photobucket.com/albums/t41/apistomaster/P3220002.jpg
This domestic pair had to use the cone or the glass; they chose the cone.
http://i157.photobucket.com/albums/t41/apistomaster/P8130031.jpg
Sounds like a pretty anthropomorphic theory to me.