Discus are schooling fish and for the general well being of this kind of fish it is very strongly advised that they be kept in groups. They need a tank of at the bare minimum 45 gallons and a bigger tank is always the better option for them. Since they can grow to the size of a small dinner plate, a large aquarium is needed. They thrive on super clean pristine water so water changes and lots of them are required to keep them healthy and happy. Juvie discus need to be fed several times a day and generally require a bare bottom tank. They can and do waste away in an aquarium that has substrate or one that is planted. Sub-adult or adult discus can be fed once a day and can be kept in a planted tank or one that has substrate.
So in answering your question I would have to say it is not a good idea to keep a single discus in a 30 gallon tank that is planted.
Marinemom