All flushes go to the ocean, right?

I think I just failed one of the major life lessons I was supposed to teach Logan.

For Logan's 4th birthday, he got 4 fish.
He named them
-Fishy (original, huh?)
-Zak
-Zoo
-Za

The last 3 were zebra fish. They all looked the same. If one of them was being naughty or whatever, it was usually labeled Zak. Zak was the naughty one.

Well, as the 6 months went by, Zak grew very long. (When I say very, I mean like 1 1/2 inches) Zoo grew fat. Like pregnant looking fat. Za didn't really get long or fat. He appeared to be the only fish that ate until he was full and then swam away.

Things went fine up until a couple of days ago. We noticed that Za was looking a little thin. We noticed that when the food was dropped in that he/she didn't eat.

We thought that it was probably going to be her time soon. As sad as that was, we always told Logan that fish don't last long, and he would have to learn a life lesson about life and death.

Well, last night we noticed something awful. Za was missing 3/4's of her tail fin and half of each side fin. Like they had been CHEWED off.
While I was reading Logan his bedtime book, Dave had his eye on the tank.
Freaking ZAK was picking on ZA.
I left Logan's room feeling SOOO awful. I didn't know if we could help Za, seeing his fins were basically gone. :(
He was still swimming though, so that at least was good.

This morning, when Logan woke up I told him to open his blinds so the fish could get some sun. He told me that Za was stuck in a plant.
Sure enough, I went up and saw that Za had no more fins and he really was stuck.
It really was awful.
I told Logan that it was time for Za to get flushed to the ocean so he could have another NICE fish help him find some new fins. He cried a few tears, but thought that Za had to get away from Zak.

Dave did the job, and he thought maybe Za was already dead. I don't know. To me it looked like he was still 'breathing'.

I feel so sad!
We flushed him.
I figured it was a way better fate than leaving him there to be attacked until death.

Right?
Did we do the right thing?

And seriously, what the hell makes 4 fish get along great and then 6 months later one takes over and kills another.

I try to tell myself that it is all nature, and the strongest survive. I never wanted these fish anyway, so why did I feel so bad.
I was cheering on the 'GOOD BYE ZA!! Have a good swim in the ocean!" Logan was happy and clapping. He even flushed. I was fake smiling and wishing Za a safe journey!

If Za had just died, we would have told him he died. Seeing he was de finned by another one of his fish, we just couldn't.

Mean Zak!