i have 2 tiger oscars that are about 18 month i figure and they're between 10 and 12 inches.
as for introducing a new fish into an established tank it is always a hit or miss kinda thing. it comes down to your fish's personality and how he gets along with others.
as for why your oscar is smaller than he should be it could be because he was stunted in a tank that was to small for him, poor water conditions or maybe he's just a runt. it really is hard to say for sure.
hope they start to get along soon...good luck to ya

Hi guys and girls, Well i reintroduced the fish yesterday morning and things were looking great. The went all day without fighting. However around 8.30 i was watching telly when i heard the water erupt! My oscars went berzerk and now the smaller one has a mark on his head! Ive divided the fish up again and am going to give them another go later on today. Would it help if i put them both into a new tank which is unfamiliar to both of them as my smaller oscar seems to be being very terratorial about his tank! thanks again...

if you have got another tank that is big enough then yes, Try to put them in it and see what happens. It could well be that they settle down because they are both In unfamiliar surroundings as you have said. However, it may just start again once they have settled in, if one of them just doesn't like the other one, though not suddenly going to become mates just because you put them in a new tank. Oscars can be funny fish, you get some that are very placid and non-aggressive, others are just plain wicked & will pick on anything. But, anything is worth a try and it might just work. I hope it does, it'll be some good useful information to add to the website

Im currently trying to source another tank in the next few days so once i have found one and got it set up and ready ill put them both in at the same time and see how it goes! finger crossed! A friend of mine has just bought a very large tank... i believe its around 500 litres. However he wants to buy between 3-5 babay oscars in the hope that they will live together when older? is this possible? Thank you

if you are just cycling a tank amendment, I'll be very careful about putting two Oscars in there straightaway. The biological filter in the new cycled tank is very delicate, you cannot just add big messy fish straightaway, you'll just end up with poor water conditions. have you got any other fish in with the Oscars at the moment? If not, can you just swap the filters over?

500 L is about big enough for two Oscars, it is not big enough for any more so make sure your friend knows that

my oscars are on their own so i could swap the filter also?