I have 120 gallon tank with an Oscar (9"), a JD (6") and a Pleco (10.5"). I am wondering:

1. Is the Oscar going to be miserable without another Oscar?
2. Same question regarding the JD?
3. Do I have room for another fish anyway? Knowing these guys are not fully grown and that I will NOT be getting a bigger tank?

Advice please!

Brooksie

i do not think fish are miserable when kept alone.
They will react different when alone, but i do not think fish can feel lonely, like us humans.
I've seen different behaviour in my Oscar, with different tankmates and indeed after my green sunfish died, with which my O grew up, my Oscar seemed not so happy anymore.
But with the Vieja that bullied him he didn't act like his old self.
After i rehomed the Vieja, my O seemed very happy.
And as you know i got another oscar and the 2 of them love eachothers company.
But i'd say leave the situation as it is for now, but that's more due to tanksize.

adding any one fish would change the situation... if any two would pair up,
the other kind would be ganged up.... that is the only concern since tank
size can still accommodate another occupant....

introducing a new fish into your tank with the current maturing stock you have is a questionable undertaking. there is not guarentee on how your current stock will take to a newcommer and since you don not plan on any upgrades i would think long and hard about it.
also like noddy mentioned you are almost at your limit, tank wise, with fish you have now and maybe it might be best to just give your current stock the space and reduce the maintenace.......

Oscars and jd are not schoaling fish. they like a territory so will be quite happy with on their own with their own territory 'waiting' for a member of the opposite sex to come along!

Schoaling fish like to have fellow fish with them as they use the other fish as a early warning sign of danger

I do believe that I will stick with my current tank set up... I have had problems with water quality in the past before but things are now under control. In other words, don't poke the sleeping bear!!

Thanks all !