I undertandi dont have larger tank at the moment. and by "100+ " i meant a tank that has a capacity over 100 gallons. i want to get a tank that is closer to 200gal though.
While Oscars are notorious for sulking (I know, I had one too) i would test the water just to rule out the possibility that you may have had an ammonia spike or something of that sort. Is the current 10 gallon cycled?
How long do you plan to keep the 40g running?
It's pretty common for an Oscar to hit 8-10" in its first year.