I've been thinking about adding a "shallow water" plant to my goldfish tank - something with roots in the water, and plant growing out the top. The one that comes to mind is a calla lily, though I am very open to recommendations, as I know nothing of water gardening at this point - including what might be poisonous to the fish.

I am thinking I could start it in the fish tank, and then bring it outside later - when the fish also go in the pond.

Does anybody have any thoughts on this?

Water Lettuce is good it also sucks nitrates up double plus, the only catch is it grows so quickly that it is illegal in most states in the US not all have outlawed it so you may be able to get some. The goldies will most likely eat this one but duckweed is fairly pretty.