We recently visited & overall welfare seemed low, particularly in relation to the elderly animals which were housed singly with no ethical explanation given when we spoke to one of the keepers
In fairness to the zoo, if they have a pair of a species and one dies of old age, what do they do with the other one? With a lot of ageing animals you're going to end up with a lot of single individuals - and of course if you get a new companion they may not get on, or the remaining 'oldie' might pass away soon after leaving the new arrival on their own! If there are other genuine welfare concerns then fair enough, but that alone seems a rather harsh thing to pick on.