I'm planning on having between 20 and 40 cardinal tetras in my planted 65 gallon tank. I like the look of a tight school. So it seems to me I need to add a fake predator, one that will keep the cardinals' school tighter.
Is this even necessary? And what would a good extra fish to have? Would SAE's work? They move around a lot and I always seem to need them to keep the bread algae down. Note, I don't want the "predator" to actually hunt the fish, just keep them in line a bit.
For example, check out this video, http://www.youtube.com/watch?v=SmovE40nLDg
That's what I want. (But not the awful music, turn the sound down if you are at work...)
Now check out this one: http://www.youtube.com/watch?v=38fwwQezoYs
that's a much looser school. I greatly prefer the look of the first.