Water Gardening: Water garden inside?, 0 by

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Now, taking your circular saw or the cutting wheel of a dremel, cut slits into the fiberglass. I'm showing you a photo of the side of my skimmer just to give you an idea of what the slits would look like. If you use pea gravel, make sure the slits are not so wide that the gravel will fall through them. The whole point of this piece of plastic (or whatever) is to hold the gravel up from the bottom so that water can flow unobstructed underneath it. Put plenty of slits in it. The more the better, but do not compromise the strength of the piece.
Now you have to find something to hold it up off the floor of the pond. How high you want it will depend on how deep the pond is. I don't know if there is a way to figure this out accrurately, maybe someone else will have an idea. I would say if the pond is about 18 inches deep, maybe have the structure about 3 inches off the floor, with a 2-3 inche gravel bed. You might find some information about sizes of undergravel filters in relation to water volume by seaching aquarium sites.
Whatever you choose to hold the thing up, make it easy for water to flow around them. Choose something narrow, then place them around the perimeter of the thing, and also several in the middle. Make sure that it doesn't sag anywhere.