An easy way to get a Shark tank, is to get the pools from Wal-Mart and fill that up (The ones that cost 300 dollars or more, with the plastic supports). Only downside is you could only look from the top of the tank. But it gives them plenty of swim space and it's round. Another thing I would be worried about is how flimsy the pool would be.
I would do a custom shark tank only if I had a heated barn type of space. Separate from my main home, but still insulated and heated. I don't think it would be difficult to build one or to expensive. Just some plywood and concrete, only make it three foot deep. It's a dream, but might never happen. :-p I would love some black-tip sharks, but I know they need a big swim space. I know of a shop that has a 40,000 gallon tank with three in it, and that even seems small for them. They went for depth thou, I would go for length and give them more swim space than depth.
Fish Sticks, the best thing you can do with your dead fish.