Recommendation or 2 about dealing with overpopulation....

Only time will tell - it sure has. I cannot seem to be able to keep my
nitrites down any more and the silver dollars were breathing/gilling hard
before my latest water change (1 hour ago) and the fancy guppies are dieing
off approximately one every 2 days (unfortunately females - maybe because
they are large and need more oxygen which their gills are unable to provide
enough of because of the nitrite inhibiting their gill action) occasional a
male passes away, and today the silver dollars aren't eating and the fancy
guppies aren't eating nearly as much as they used to. I talked to my lfs
representative (don't worry about my choice of words - my lfs has been
family (not my family) run for over a decade) and he agrees with my plan to
buy an oscar for $9.00 to eat the fancy guppies (all of them) and raise it
in this 55 gallon tank (the silver dollar tank), and he agrees I could use
other large fish like other cichlids. His bottom line is that he says he is
confident he can sell me a function fish to take care of this problem for
sure. I consider this idea a humane way to deal with the situation (it
doesn't bother me and unfortunately it is the only reasonable way I can
think of about how to deal with this overpopulation problem) especially
considering what the humane society of america does with higher animals
(dogs, cats). Of course I am open for recommendations and the oscar or other
large fish (the oscar would be sold to me at a 2 or 3 inch size which I
worry might not be hardy enough to grow through the initial high nitrites
that would be in the silver dollar tank ultimately with the silver dollars
and the one large cichlid type of fish and the tank is a 55 gallon tank -
would that be o.k. guys/gals and including those of you who are experts?
Even with the function fish I will have fancy guppies in 2 other tanks that
can be used to hopefully humanely feed as live food to the function fish
later after the silver dollar tank is "cleaned out". The other tanks have no
problems right now because one of them has 3 turtles in it that keep the
fancy guppy population under control and the other tank has only just
re-started a population of fancy guppies in it. Thanks for reading this and
please give me advice?! Thanks, good luck and later! P.s. I am looking at a
2 or 3 or less week timeframe to get the new "big" fish and I hope it works!
I will never burry any fish alive so to speak, because I consider that
inhumane so I won't throw the fish out with the garbage or anything like
that so to speak (I know....I know.... some people have a hard time
differentiating what's humane compared between burying alive and providing
to other creatures as food which is most likely a quick death making it more
human) but I have to do something or else I might lose my silver dollars and
after that one recovering from the accident a few months ago so well it
would be a shame to lose one or both of them to a dumb overpopulation
problem. Again, later!