4-6"

I did mine 2-3" and they will push it out to make a depression where the middle of the bed is back down to pond bottom.

Like JohnF says, something under it is a good idea. I did not do that. Something I think would work great if you do not mind spending the money and/or do not need to many of them is the plastic concrete hand mixing things. They can be found at big box stores like Home Depot for about 5-6 dollars each. Under water and out of the sun I think they would last for many, many years. Stack them edge to edge or spread them out with gravel in between. His carpet idea sounds like a good alternative also.


John

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