I would personally switch to a fully terrestrial tank with a water bowl. I believe that I read on CC that tigers in semi-aquatic setups, aside for breeding, are more prone to disease. And if you get another, more burrowing space would be appreciated.
If you find another it's more than likely it's wild caught, and you would have to quarantine the new one for a minimum of 30 days, I don't think mixing of subspecies will be much of an issue of quarantined, they don't breed in captivity often, and when they do it's almost always outdoors.