http://www.diclib.com/cgi-bin/d1.cgi?l=en&base=coed&page=showid&id=27697
 
Usage   
 The normal plural of fish is fish (a shoal of fish; he caught two huge fish). The older form fishes is still used when referring to different kinds of fish (freshwater fishes of the British Isles).  

http://www.eslprintables.com/forum/topic.asp?id=6193

My comment is the same as Douglas ´s. I would teach starfish but accept both.
The Australian National Dictionary says that the plural of starfish is starfishes, or collectively, starfish.

collectively: not different species.

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http://books.google.com/books?id=R9ZOlwaqumkC&pg=PA185&lpg=PA185&dq=grammar+starfish&source=bl&ots=c3B8wd6WO1&sig=1K5FO7D7i32Lch8pkFVez_-xNGs&hl=en&ei=Q_6QTZXTIMeS0QH8_PDjDg&sa=X&oi=book_result&ct=result&resnum=5&ved=0CC0Q6AEwBDgK#v=snippet&q=fish&f=false
ESSENTIAL ENGLISH GRAMMAR
In this book pg 45  it says: "In addition to these larger groups, all of wich add either s or es for the plural, ther are a number of samll groups of nourns with special plurals: ...fish - fish (that is, the plurar for fish is fish).

Then on pg. 144 the example goes: "I spent my days quietly, swimming and canoeing; I even caught a few fish. (Use a different constrution if you wish to make one element distinct from the others)

If you had caught speckled trouts, barracudas, bluefish, a bunch of triggerfish, lots of slings, a bunch of amberjacks you can say that you got some fishes (different species).

If you caught ten gaff-top catfish, it means that you got some catfish (not catfishes).

     Same to applies to starfish, you may have five starfish in your hand, and you may have five starfishes (say, one Red-knobbed starfish, one eleven-armed sea star, one Bat star, one Blue Star and one carpet sea star), then, five especies.    You stating,  starfishes, you are making the point that you have five different species in your hand, okay?

There are 2,000 living species of starfish that occur in all the world's oceans  source:   http://en.wikipedia.org/wiki/Starfish