I was recently given a 55 gallon tank which was the home of 2 plecostomus and a rope fish. They range in size from 13" to 16". The problem is I got the tank for my Oscar who is 11". Can I have all these in the tank together comfortably? They have survived 24 hours together but the Oscar has lost a few scales and is swimming erractically; at times he appears to shake. What can I do? Does he need treatment? I have had him for 2 years and don't want to lose him.

first thing first... i am not sure about the rope fish but
55 gal is good for one grown up oscar only. 2 plecos at that
size is already a messy creatures and will add loads to your
tanks.

when you move your oscar in the new tank, is it properly cycled?
where is the former tank of your o. have you used the existing
tank water as your seeding water for the new tank?

some hobbyist commit this mistake whenever getting a new tank.
they usually start with all new set up, without considering the
water cycle and the good bacteria colony. that is where problem start.

Thank you for your response.

I did start with a fresh new set up. As for the old tank (a 20 gal) its the new home of 2 baby Jack Dempseys for now. I will keep close eye on the 55 gal and its inhabitats.

shaking perhaps is due to new tank syndrome. the new
tank is not fully cycle yet.

can you check water parameters? i suggest you barrow half
of the water from the old tank, half of the filter media,
to seeded the water and the good bacteria in new tank. make
it fast to avoid killing good bacteria in the old filter. it
will help fast track the cycling of the new tank water.

paul is right, your tank wasn't quite ready for fish. oscars are sensitive to ammonia, and with a new tank and big fish, your just begging for extremely high levels of ammonia. do exactly what paul has recommended, but one thing i'd add isto feed as sparingly as possible. less food = less leftovers and less fish waste. get that kit, if your levels are out of control, your only option may be to resort to chemicals. which id try to stay away from. let us know how things turn out for you.

btw 55 gallon is not big enough for all those fish. and the 20 gallon is also not big enough for the jacks.