This is something crazy and new for me! My family, the Bradys, have an aquarium and it was low on fish, so I stocked it with an Angelfish. It already had a Deathfish in it and as soon as the Angelfish went in, the Deathfish attacked and killed it! I could see a ruckus going on inside the tank and got a few screenshots, then the Angelfish was just gone! I put a second one in, but nothing happened. I guess it's something random or maybe the Deathfish doesn't always immediately attack. I've had aquariums myself in the past and I know that some fish will fight with and kill each other, but I never dreamed it would be in Sims 3!
Here's the best of the screenshots. In the middle of the tank you can see a cloudy area with a star, #, and ! sign to indicate the ruckus and fight. The Deathfish is near the upper right of the cloudy area and the Angelfish is directly below it.
Update: The second Angelfish is now gone from the tank. I'm wondering if it's an issue of Angelfish being bait for Deathfish. And if so, will other fish fight and kill their bait if both are in the same tank? I'll have to test this. Because there's no body to scoop out of the tank, I guess it's safe to assume that the Deathfish did indeed eat the Angelfish. Furthermore, if the fish get fed often enough, would the Deathfish leave the Angelfish alone? Hmmm...