I started this thread and would like to summarize what I have found so far.
First of all, no one has answered my question directly, evidently because no one accessing this thread really knows. So I'm still waiting.
From the suggestions in this thread and from what I have read on the Internet, it seems that the best Aquarium DVD for a 42" ED plasma currently available is "The Complete Aquarium Collection" -- a 2-discs set dating from 2001 that includes five different aquariums. Price ranges from $18. on E Bay to $23. I've read all the pros and cons I've been able to find and there is no complete agreement, but the pros tilt the balance in favor of this set.
SereneScreen probably has the best PQ on the subject, but no widescreen DVD yet.