nope, you got it right!
Beautiful sand dollar by the way, I don't think I've ever seen one in a tank

Thanks everyone for the info. I'd like to have one to help keep the sand bed clean. We have a yellow head sleeper goby who does an ok job but I think he could use some help.

He won't be of much help. I'd say they're more ornamental then anything. They do clean but at a very very very slow pace. Add too many and you run the risk of starving them.

If I get a sand dollar I will be only getting one.

Where in Edmonton can you get sand dollars and how much are they? Do you think they can be in a tank with a sand sifting starfish?

I wouldn't recommend it. Since both target the same animals for food in the sand, and since our tanks aren't huge enough to sustain a large, diverse sandbed, either one, or possibly both, will eventually starve.

Do you think one would starve in a 230g? I don't like the idea of buying one if it's slowly going to starve to death.

That's a hard question to answer. Factors like age of sand, depth of sand, etc play a huge roll in things. However, I haven't really seen very much information on long term survivability of any of the sand sifting animals. Aside from the one poster who has had his for 2 years, I haven't heard/read/seen anyone else with one that has lasted more than 1.

The sand is half new and half a couple of years old (mixed old sand with new when we started up our 230g) and is about 2 inches deep. There seems to be lots of worms etc. in it for one to eat. Thanks for the info it's good to know.