I believe this movie was made sometime during the 90's.  It might be a
Canadian or a French movie.  In the movie, a photographer and his wife
go on vacation and ask another couple to house-sit for them [in
particular, to take care of their aquarium].  When they return, the
fish are dead.  The other couple doesn't move out.  Eventually, the
photographer's wife leaves and he is left alone with the other man who
is becoming abusive.  In the end, the photographer places that other
man into a wooden box [which allows the man inside to drink, and
allows the photographer to see and talk to the man inside via a CC
TV], and watches him die.  What was the title of that movie?

Dear billbauer-ga ,

From your detailed description I believe the film is Cleopatra's Second Husband.

A synopsis of the film which matches your description appears here and
a specific reference to the box appears in the review below.
http://www.rottentomatoes.com/m/cleopatras_second_husband/about.php

?Robert's subservience, and Zack's domination of him, takes a sudden
turn when something snaps. Robert has had an obsession with doing a
photo study of captive insects. He decides that Zack would make a good
subject for his study, drugs him, then, locks him in a box to observe
his death. The sheer creepiness of this action causes the story to
take a bizarre shift in direction that actually helps the flick as the
worm, Robert, turns.?
http://www.reelingreviews.com/cleopatrassecondhusband.htm

Internet Movie database entry.
http://www.imdb.com/title/tt0146562/


I hope I have correctly identified the film. If I have not, or the
answer is unclear, then please ask for clarification of this research
before rating the answer. I shall respond to the clarification request
as soon as I receive it.
Thank you
answerfinder


Successful search strategy
aquarium "house sitters" movie, led me to the film?s name.
"Cleopatra's Second Husband"
://www.google.com/search?hl=en&lr=&q=%22Cleopatra%27s+Second+Husband%22&btnG=Search

Awesome!!!!  Thanks!