First thing, the iCloud and Apple Game Center are 2 different formats. Game Center is primary and backed up by the icloud. Saving stuff to the cloud is Good for photos but not Game Center. That's what your Apple ID and passwords are for. Apple save all account info for your games not the freaking cloud like people think it dose, you don't store passwords or account id in the cloud, if you do your telling everyone what they are.
To recover your tank, set up the new hydrus account you found when you logged in and found your tank gone. go thru the tutorial play one game. Then go to the options tab then the account tab it should only show the hydrous and a note bind this account to Game Center. Open your Game Center in your settings log in to Apple with your id and Apple pass word. This make your Game Center active.
now go back to your iron force account page, you should see your new hydrus account and active
below it should any other tanks associated with your Apple ID So select by highliting that tank and select load to this account.
you then make that tank the primary tank and the hydrus inactive. It going ask if it's ok to override the hydrus data, say YES
your done, your tank has been rrecovered. Leave the hydrus account alone it linked to your tank or tanks.
like I said this only recovers a legit tank, not a third party purchase tank from a buddy or internet.