My son is very matter of fact about death. In his 4 years, he's lost a special fish (tons of personality and demanded attention, even from guests) and a cat. He really didn't react to either. With the fish, he joined in the "what do you *do* with a fish this big??? It can't be flushed!" (it was 15 inches long), but didn't really talk about it having died until later. With the cat, he verified a couple times, "she not come back?" then let it go. (He didn't see the body in that case.) Now, months later, he'll mention one of them and say he misses them, especially the cat, but it's still pretty emotionless. I think, at his age, he really doesn't fully fathom death.
Also, not to be callous, fish don't usually live that long, so I don't tend to get very attached to them. Generally, for me, a fish is a fish. I like to have a betta, so when one dies, I go and pick a new one. I've only been truly upset over two fish. Both of the fish I was upset about had lived for years and had a lot of personality and interacted with people. One was my little runt of a betta. He was friendly and I was his person. He liked me better than anyone else.
The other was the one my son knew. He was a big old pleco named Charlie. We'd had him for 5 years. He watched tv, threw fits when he didn't get his food on time, raced to the front of the tank when someone approached... He was more like a cat than a fish.
In my son's life, though, we've lost a lot of fish. Some I had before he was born, some weren't healthy when I bought them and just didn't live long. I'm generally happy to have a fish live a year. I don't bond with them like I do a cat or dog because I know I won't have them long and I can't touch them. So I wouldn't necessarily be concerned about a lack of reaction to a fish's death, anyway.
