Pure aluminum is safe. But many aluminum objects are actually made of an aluminum alloy. Many alloys may be unsafe for fish.
They wont harm your fish.
Pretty sure James means Pilea Cadierei (ie Aluminum plant) and not a plant made out of aluminum. If you keep only the roots in the water, it might live with enough light, but it most definitely won't survive underwater. Less reputable dealers sell it as an aquarium plant because it's pretty and takes a long time to die underwater, but it's not aquatic (you'll never find it at Aquarium CoOp, for example)
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