I got a baby Oscar (6-7cm) on 31st July (Saturday). I had him in an iso tank (inside the main tank) for week because I wanted to make sure that he got settled & was eating properly before I let him out into the general population. I've read about how Oscars can be moody & sulky if they go through a big change like a tank change & I figure being pushed around a pet shop & then arriving at a new house would be a "big change"

Now, he was settled & eating & seemed happy enough & I let him out into the general community on 7th August (Saturday). He was fine over Sunday & Monday but then yesterday (Tuesday) he seemed to be moping around the back bottom corner of the tank & wasn't coming up to get his food. When he did come up to eat a "little bit" the other fish (a bunch of 6 Mollies) would come over and my Oscar would just back away.
Today (Wednesday) he was just sitting plopped at the bottom & didn't even try to come up for dinner.

Okay... so I've now put him back into the iso tank (inside the main tank) & he immediately perked up a bit & started eating a bit.

Now, the question... yes there IS a question. I'm pretty sure that he's a bit daunted by the big tank & the other fish, & I have a Redtail Black Shark who tends to chase a bit. He doesn't chase badly & he's NEVER hurt anyone (there are no signs of physical damage to my Oscar) & I'm pretty sure that the shark has chased the baby & scared him.

My plan is to leave my baby in the iso to settle & calm down again and maybe in another week (or 2 or 3... hmmm) try him in the tank again.

What do you guys think & what is a good way to introduce him into an established environment.

Now, some details.... yes, I have a Redtail Black Shark (10cm) & yes, I know they're territorial. I also have a Black Ghost Knife Fish (25cm) & 2 Plecos (10cm each) also 6 Mollies.

The tank is 320 Litre with a 1200 L/hr canister filter. The tank is roughly divided by plants into "areas" so the fish can define territories & the tank should be large enough for the Shark & the Oscar to stake out seperate areas.

Okay, sorry about the long winded essay but I wanted to give a good picture so I can get good answers. What do you guys think is a good way to help my "Scaredy baby" to join the Redtail & stake his territory without too much drama. The shark really doesn't chase very much but I think my baby is just a bit of a chicken right now. Maybe I need to keep him iso'd until he grows bigger than the shark

I am not as confident as you that all fish have enough room as the knife can reach 12"+. as they are small you should be ok for a a while. you have 3 "night fish" so the bullying could be at night. any smaller fish you put into a new tank is going to feel threatened. it usually lasts a few days and if you have enough hiding places they will adjust ok. if there is not any damage you could just wait it out. but tail nipping may show up after it has happened.

i only have expierience with my baby Oscar coming into his new home, where i allready had a small sunfish, some silver dollars and some plecos.
But he felt at home from the moment i put him in there and only started sulking when i did my maintenance.
Just keep a close eye on the other fish and see who may be spooking him.

An oscar needn't be bitten to get spooked , especially at night.
My oscars can go totally nuts when only touched the slightest bit by another fish at night.

On the other hand dither fish like mollies, should give the oscar the idea that things are fine and safe, if they swim around freely.
Just give your baby some more time.

As long as the oscar hasnt got injured then i would put him back in the main tank and wait for him to settle down

Your tank isnt big enough for all your fish although the mollies will end up as oscar food when he gets bigger. Also your oscar will have great fun ripping your plants to shreds and uprooting them