PURA NitrateLock 500 ml. Selective Ion Exchange Resin For Nitrate Removal. PURA NitrateLock rapidly removes and maintains extremely low to non-measurable levels of nitrates in fresh and salt-water aquariums. Long lasting, can be regenerated up to 24 times. Drastically reduces number of necessary water changes. Regeneration is accomplished with plain table salt, quick and completely safe. 500 ml - treats 50 Gal tank for up to 24 months.