30 high or long? they are large active fish and would be best off in a long tank with others of its own kind. However, it would probably be better off in your tank than in a "poorly maintained" one. For the long term plan, I would recommend getting a tank that could house a good sized shoal with plenty of horizontal swimming space. At a guess, considering their adult size, I would say you're probably looking at a 55g long. Short term, it can probably stay in your tank for the time being, though I would be concerned that it might bring disease in to your tank. Quarantine it, or be prepared to fend off some illness or another.
Another option is to bring it in to a petshop. Some stores will take in unwanted fish, though best to call ahead and see if they will. Even better if they have other giant danios in stock so this fish can be with its own kind.