Clay bricks are safe. They are basically fired clay. Be careful if you will leave them over winter because they may crack into pieces due to the water in them freezing and expanding.

You can use the bricks but they are real hard to move around. I use cheap Sterilite stackable plastic crates to raise my lilies about 12 inches and it also serves as a place for fish to hide inside should they want to. Cost about $3-5 at any big box store.

i used regular building bricks the small red ones with holes through them. my fish are happy and well:)

Hi
Of course you can use bricks even concrete blocks .As frogman said they are tough to move around I use old flower pots because as the plants grow you need to keep lowering the depth. Another good adaptable cheap alternative is "laundRy baskets" full of holes so the fish have a place to hide as a perk . gary

I use plastic 'milk crate' type crates with an overlapping flat rock on top. This keeps the crate in place and keeps you from seeing the crate. The holes in the crate give the fish a hiding place if anyone comes fishing.