gravity always wins

I quit creating slopes years ago for that very reason.
 
Nothing you can do long-term. Sand flows and gravel slides down too. You'll have to bank it. Instead of buying a ready-made retaining wall, you could use slate or a length of flexible plastic. Coat the visible side with silicone aquarium sealant and smush handfuls of the substrate you're using into the silicone while it's still wet. This would give you a wall that pretty much disappears into the background or behind foliage. (Let this cure for 48 hrs before placing in the water.) You could also poke some holes into the wall to add some moss or the smaller anubias that would slowly grow from there. A single branch of driftwood could be covered the same way.
 
I tried a slope but my pictus didn't like it and smoothed it all out again. He's a jerk like that.
 
I have put gravel in bait bags from the bait shop, and covered them with more, loose gravel.
Works out pretty well.

Or you can buy a fake Loufa from the store, and cut the string that holds it together, its a fine plastic mesh. Use it to hold material in place, and then cover it with gravel to make it look natural.
 
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