Pete R. , im in full agreement with lando for even if you went with a 500 gal
tank, you still be way short from what you would need here and your thinking on a pair of leopard sharks.
To give you an idea on what you be looking at, i will give you something on what i done so long ago.
I had setup a shark tank that was 15' long and nearly 5' Wide and all together was a 2500 gal
tank. Under this i had three huge sumps and two on both ends of that tank with a huge skimmer and a 500 gal
sump for L/R. And in short, this 2500 gal
tank wasn`t suited enough for the life span for it would need to be no less then a 5000 tank or pond and yet if you can do, go larger.
In a shark tank, your filtration and currents is very important and you would need to do twice the normal water changes you do now. If you like to talk to another person who knows very well in what is needed here? Go to http://grimreefers.com/
and go to the shark & rays forum and ask mattiej If anyone i know in any of these forums who is well off suited to answer any questions you might have, it is he
And tell him that DME says HI