Many of us didn't know about the nitrogen cycle when we first started, so you're not alone.

Since you'll be starting a new, it's very important for you to get an understanding of it.
Here's a basic explanation:
First your ammonia (from fish waste and left over food) will rise.
In a few weeks bacteria will start to develop and you'll see the nitrite levels rise and the ammonia levels start to drop.
After a few more weeks a different kind of bacteria begins to develop and you'll see the nitrate levels rise and the nitrite levels drop.
Ammonia and nitrites are toxic to your fish.
When you having readings of 0 on both ammonia and nitrites with some nitrates showing, your cycle is done.
The link in post #2 explains it more thoroughly.
Usually fish stores will test the water for free. If you can, take the water to get tested. Ask them to write down the readings for ammonia nitrite and nitrates.
Better yet, you really should have your own
test kit. The
API master kit is highly recommended. The strips can be inaccurate.
I don't know that much about Green Terrors, but from what I understand, it's their nature to be aggressive and territorial.
If your Oscar was out and about before you got the GT, that's most likely whats going on.
Nick gave some good advice. Unfortunatly sometimes we get fish that just don't belong in the size tank or the fish we have.
It doesn't help that fish stores don't tell customers any different.
Edit: Pretty tank
