substrate: play sand very deep .. ?

I don't see a way to do what you want that will sustain that slope over the long term. Although, terracing the grade might help get you close to the overall effect you want.

Keep in mind those gorgeous planted tank shots we find like that are meticulously created and groomed to achieve the most perfect photographic moment possible by the aquascaper. The tank is displayed or the photos submitted in competition. In the end, tanks like that are seldom maintained the same way after that.
 
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