It depends on where you live and your water change plan. Aquatic life is fine if you keep up on water changes. Jake Adams at ReefBuilders only uses a carbon block (no RO), but he lives in Colorado. Perfect 0 TDS starts to matter most with big tanks that may not do frequent water changes and are throwing in lots of top off water all the time, so small impurities and contaminants build up. I feel like people here are overgeneralizing the need for high end RO/DI units.