Why do you insist on posting and encouraging erroneous information? For Pete's sake, why? You're not helping anyone. You're hindering others in learning and at the same time condemning fish to lifelong cruelty.
Please, do tell me on how you can tell your fish are happy. Did you ask them?
That's literally the biggest BS you can use as an excuse..."my fish are happy"
Abused dogs will still wag their tails after their owners beat them
A captive dolphin still has that "smile" on it's face even if it's living in a small pool
A bullied child will still act "happy" on the outside, even though on the inside they are miserable
You simply cannot judge a fish's "happiness" You can provide for their needs, and hope they are complacent with their lives.
In this case, you need to provide an Oscar with a 75g MINIMUM.
Your oscar can "swim normally in their tank just like anyone else's tank"
Ok, well, going by that logic, your Oscars will be very "happy" in my Fluval Chi 5g. I can tell, trust me.
"However, do some reading fish do grow according to their tank, and there is acual evidence of this"
Of course there is evidence of this. Wanna know what it's called? It's called stunted growth. It results in a misshapen, tortured fish who lives in constant pain and suffering. There is no cure or treatment. Except giving them a bigger tank before they are stunted.
What you are attempting to do is like trying to raise an alligator to live solely in a bathtub. It simply isn't going to work.
Funny on how you chided DM for poor grammar, but I see an astounding rate of grammatical errors in your posts...
"I own about 300 fish"
Do you now? What, did you stuff them all in a 20g?
There is absolutely no reason or excuse why there are over 100 fish in a 10g
It doesn't matter if they are fry, guppies, or whatever. It is absolutely, unnessecarily and downright cruel to have 100 fish in a 10g.
"I know very well when a fish is being cruelly treated"
No, you don't. You absolutely cannot. Otherwise, you would not be so scarily OK with keeping 100+ fish in a 10g, JDs and Oscars in a 55g, and telling others an Oscar will be fine in a 20g.