Let me start by saying, I never intended to own an Oscar. The one I have now was rescued, it is 3 inches long and it was in a 30 gal tank.

I set up a 55 gal tank, the water perimeters are where they should be (i had a seasoned Oscar owner check for me) the temp is perfect, the exact same as the tank he came from, all equipment (heater/filter etc...) is working as it should. I transferred him into the tank and he is now hiding in the back darkest corner, which I'm sure is to be expected.

How long will/should this behaviour continue before I get REALLY worried about him. I have grown quite attached to him and want to do everything possible to give him a long, comfortable life.

FYI I did use stress-coat after the transfer, its only been about 10 hours as of this posting.

Thank you

You say you set up a 55 gallon tank? did you cycle it before putting the Oscar in? Obviously if you set it up and then put the Oscar and straightaway, the water is going to be okay. However, give it 24 hours and you may find that ammonia is present. If however you did cycle of tank then just give the Oscar a little bit of time to get used to his new environment. Then turn the lights on for too long, they feel safer when the lights are out. Give them somewhere to hide, a big rock, or nice piece of wood is often a good place for them to hide behind.

If you didn't cycle the tank before putting the Oscar in, I would suggest getting your own test gets, start testing for ammonia and nitrite every day. Go to the section on helpful articles and read up on cycling the tank, or have a word with your friend, he'll probably help you out.

The tank was set up, sat running empty (no fish) for 48 hours and then was tested before he went in.

if you used the filter from the 30 gallon tank you should be alright but if you didn't you will probably run into ammonia and nitrite problems. as ofl said it takes weeks to cycle a tank and a testing kit is essential. if you do start getting an ammonia reading i would recommend doing a 30% water change every second day until the level falls to 0. the same will apply to any nitrite readings you may get. i would also recommend getrting an liquid testing kit not the strips. i myself use an api (aquarium pharmaceuticals) kit.

fyi an acceptable water reading for oscar tanks is ammonia 0 nitrite 0 and an nitrate reading below 40.

good luck with your new oscar