Is there a size/behavior difference in Silver and Black Arowanas?
Hmmm... I think there is but I'm not sure everyone does. In my experience the silver arowana often has a meatier appearance. When you look down through the top of the tank the silver's back is often wide where the black is slightly more lithe and moves in a slightly more elegant (if that's the right word) manner. It's also been my experience that if you have a tank large enough to grow either to full size the silver will probably be bigger than the black though both will get around 3' or more as full grown adults in an aquarium. The silver maybe a bit longer. Generally speaking the black arowana will have nearly the same coloration as a silver once it's 6-8" long but the black (edit) will have a slight sheen of something else as well. A little pink, a little blue, etc. but only slight.
I also think the black is a hair more skittish but again only marginally so. Both have the same inherent nature which is to make a high drama escape when startled. Sometimes that's just a huge splash. Other times they're heading out the top. Many fish have identifiable tendencies that you can recognize quickly and a random example would be that Peacock Bass have was appears to be a dorsal fin right behind their dorsal fin. When you see that you know the fish you're looking at is going to be an extremely fast fish because he's not just using the forward dorsal like a keel to keep him in a straight line; he actually pushes with that second (adipose) fin.
When you see a fish like an arowana it looks to have almost a single fin that wraps around 60% of the fish's body. That's a fish that can develop amazing thrust for a short distance and that distance can be straight up.
My largest tank has a young black arowana in it (roughly 2' long) and I've heard him hit the lid several times. When they get jumpy it won't usually be a single jump... it's more likely to be 5 or 10 jumps in 5 or 10 minutes. I have a piece of 2" thick solid foam insulation atop the tank and it's held down by parachute cord that's anchored to tank plumbing on either side of the aquarium. In years past I've used a few 2x4's on top and other tricks to keep them in. The foam has mildew inhibitors in it but it doesn't seem to effect the tank stock.
