I do facilities work at a large veterinary hospital and I'm troubleshooting an odor that's coming from one our toilets. It's almost a sewer smell but not quite the same. After checking I found that it's not coming from the base or from the inside of the bowl. It is very concentrated in the actual tank (as unusual as that sounds). I've already removed the tank and the pressure tank from inside of it (It's a Flushmate equipped toilet) and cleaned and disinfected the inside but the smell has returned after a couple days. Any ideas of what might be the cause?