Bottom posted.
Slow down. I find it hard to believe that you can't find a store that
would take a batch of healthy, locally-raised guppies for free so you
can clear out your tank.

Do you actually want an oscar? If you do, go for it. Remember that
an oscar is not a swimming trash dump or "function fish". It's a
huge, long-lived fish that requires a considerable commitment of tank
space and care. You'll have to be careful in your choice of
tankmates too. The silver dollars will probably be OK and you might
have room for something like a firemouth, depending on how many
silver dollars.

Also, you have turtles that eat guppies so you have a way to get rid
of the gravid adult females. Plenty of other fish eat baby guppies.
Put the females in the turtle tank. Add an angelfish, a few large
tetras, or a couple of loaches to the 55 gallon and you'll have
nothing but males soon enough.

I like altum's recommendations the most. What to do about the water quality?
Well the nitrites and nitrates are high even after changing 40% of the water
every other day up until recently (the nitrites and nitrates are still sky
high but I stopped changing water as much recently since behaviorally all of
the fish WERE doing fine). I must have lost that one angel recently because
of the high nitrites/nitrates, which means I might NOT be able to
successfully add a function fish anyways? One thing about the turtle tank
though is the turtles are a small species that I have and they don't eat
even close to ALL of the guppies in their tank so I can't really get rid of
upcoming guppies. All other water characteristics are fine (soft water (I
know that guppies prefer hard water but mine have lived at least 1 year in
water from the same dechlorinated tap water source so they should be well
adjusted by now), non-alkaline, ph 6.5, no ammonia, the rest I said before),
high nitrites or nitrates HAVE to be the cause of the deaths in my opinion
and I can't seem to be able to reduce them at all at least with the current
occupants. I have a lot of babies in the silver dollar tank that can grow up
after otherwise eradicating them, to be male or female fancy guppies. I am
getting very weary of picking out fish corpses every other day so I am going
to try something drastic most likely and take in what I can of the fancy
guppies in that tank and use it towards a large fish like an oscar. I can't
figure out why the fancy guppies die slowly and I want to do something that
works. Netmax has my concerns in mind as well as if I introduce a 2-3 inch
oscar as my lfs offers then the oscar could die off soon after from whatever
is killing the fancy guppies. It just HAS to be nitrite/nitrate poisoning as
with both my silver dollar tank and my bedroom tank the fancy guppies die
off after overpopulating - summary: I have seen this cycle before. Sorry
netmax if you are getting sick and tired of dealing with my silver tank
problem (I have been trying to figure out what has been going on with it for
6 to 12 months now ad I still haven't figured it out, as I have posted many
times about my various potential answers to this problem) but I want to
treat my pets RIGHT and not leave them to die when something just HAS to be
going on that I have to figure out. Even if I got an oscar and it survives
to fix this problem I would even be able to feed it live food in the form of
fancy guppies from my other two tanks (I have a lot of fancy guppies in my
bedroom tank and turtle tank) and I will still have a lot of fancy guppies.
Sorry if I talk too much guys but this is important to me, my fancy guppies
should not be dieing off like this and I add a tsp. of salt for every 5
gallons and I have plants. Problem just keeps coming back. I even tried a
full treatment of formalin with this tank and no real difference. I guess it
all boils down to what I WANT to do more than what I CAN do, like netmax
says. Suggestion box is still open of course but I guess I will have to plan
for myself. Good luck all and thanks for reading and recommendations are
still welcome of course. Later!