My Oscar is Stunt. :( Wonder what else I can do?

Hello :)

I did a lot of research online and went to experience fish keepers near my home and talked to them about my Oscar.

Sadly my Oscar is stunt growth. She is about 8-9inches maybe and is a full adult.

She has not grown any in the few months I had her (got her this summer) so I was a bit worried. She is in a 75g tank, canister, and an HOB filter. Tank is fully cycled. I do a 50% water change once a week at 10ppm of nitrates.

She was adopted from my LFS. I knew that there were possibilities that she could be stunt. I did not know anything about her past before I got her. All the pet store said was that she had laid eggs and that they were happy to see her go as they were having issues with her because she is an aggressive fish.

My fish peeps here in CO suggested I get this for her and that she might grow an inch or two for the rest of her life which is not bad at all. I have already ordered online should be here in the next few days: http://www.drsfostersmith.com/product/prod_display...

My question is why does growth stunt happen?

I am aware that tank size is the biggest concern, but does water quality have anything to do with it? If the water was not changed can this stunt a fish? If Nitrates were high and no water change was made can this effect the size of the fish?

When I got her she did not have fin rot or hole in the head or any big injuries so her previous owner was at least doing something right. The pet shop had her over 3 weeks or so in a quarantine tank.

Regardless I love my Oscar and will keep her until she dies of a natural death. For Christmas I have a Santa boot for her near her tank. I think I will go crayfish Christmas shopping!

Happy Holidays guys! Thanks.