HI Al, that sounds like a good idea. I used the 2 Black longspine urchins. I just removed them from my tank. They got to big after some time. They would knock over corals and I found one eating my acans
It was fine when smaller but mine got bigger then a soft ball. In a dedicated tank to the cardinals, I think it would be perfect. Just be aware, it is venomous. I had been pricked a bunch of times with no reaction, but you never know? This is the info on it.
The Black Longspine Urchin, also known as Long-spined Sea Urchin, has a body which is predominately black with a red eyespot in the center. The spines on these urchins are long, thin and completely black. They are a very delicate looking urchin, which are actually very capable of protecting itself. These urchins are excellent algae controllers for an aquarium housing aggressive fish.
Provide an aquarium with ample hiding places and room to roam. Being a nocturnal creature, the Black Longspine Urchin will hide during the day and only come out at night to forage for food such as algae and seaweed. These urchins are an excellent algae controller for an aggressive aquarium where other invertebrates would be eaten. When approached by a fish, these urchins will sense their presence, and will defend itself by directing its spines towards its offender.
It is very sensitive to high levels of copper-based medications and will not tolerate high nitrate levels. If it begins to shed its spines, it is a sign of very poor water quality.
Be aware, it is venomous, with its sting being roughly equivalent to that of a bee sting.
The diet may be supplemented with dried seaweed.
Approximate Purchase Size: Small/Medium: 1-1/2" to 2-1/2"; Large: 2-1/2" to 5"