Catching a native

78 degrees is bit warm for a typical native. Not way off the chart but I try to keep my tanks below 75 degrees. Basically the no heater rule applies to all US native fishes.

As far as acclimation....Depending on where you live the water temps will be cool (40's to 50's where I live). When I take fish from the wild at this time of year I keep them in the garage for a day (about 10 degrees warmer than the water they came from) with aeration and 1 TBS per 5 gallon salt. I then move them into the basement for a day (about 10 degrees warmer than garage), then into the same room the fish will be housed. Remember to use dechlor and salt with each water change and change 100% of the water every 12-24 hours. I would quarantine any fish (store bought or otherwise) for a couple of weeks. If you QT the fish bump the salt up to 2 T per 5 gallons. If you take healthy fish from the wild you'll generally not see any real disease/parasite problem. I have just as much trouble from store bought fishes.

I would be more worried about your Oscar than the bluegill. Sunfish are very hardy and can most likely handle whatever you throw at them. The problem is they are also VERY agressive. I had two long ears in a 75. One started to badger the other to the point of death. I had to remove one and move it to an outside fish pond.

I keep native fish, and the typical "bluegill" (there are several types really, my avatar is called a "pumpkinseed") is quite aggressive. You can keep one, or 4-5, but two hardly ever works. One becomes dominant and will literally pester the other to death. In a tank with one Oscar and one Bluegill, I'll bet on the Bluegill every time.

an oscar and a blue-gill in 55 gallons? oh please say not!!!!! not only can you NOT keep these two species in a 55 gallon tank, but it isn't even large enough for a single oscar beyond a 4-5 inch juvenile. one oscar needs a minimum of 75 gallons in order to thrive ... we're talking about a fish that grows to 14 inches here.

Bluegills are REAL mean. They peck the crap out of each other and I'm sure would really **** off your Oscar...