Sorry to say it but I agree with T1KARMANN, it sounds like your keeping of them that "sucks", not the arowanas. With silvers DE is pretty much a given, either you "deal with it" or don't buy the fish. If you'd thought it thru you could have probably bought a black for around the cost of the four silvers you've gone thru and been pretty much guaranteed not to have DE. And not covering the tank is about the #1 rookie mistake, kinda hard to blame that on the fish when you should have had it sorted before bringing the first one home.
Not trying to start a war or anything, but what's the point in coming on here and "venting" to a bunch of arowana fans about how much their favourite fish "sucks" because you found yourself in two situations that should have been completely avoidable.