You cannot really go by TDS numbers either. TDS does not indicate anything in particular only that something electrically conductive is present. Even with low TDS readings it could still very easily be nitrates, phosphates, silicates, copper or any number of things detrimental to a reef system. Nitrates can be common in domestic wells and water systems that some would consider to be almost pristine.
RO/DI treatment is the best option available to us.