What are FerriTabs (Ferri-Iron Tabs)?
FerriTabs are tablets used for water treatment. They help eliminate discoloration in pools caused primarily by iron and manganese. Iron staining can appear as a coppery-brown color while manganese will be much darker and purple-ish.
These double-action tabs were formulated to restore water to its crystal clear state by removing metals such as iron or manganese. It works by removing the insoluble materials from with water by charge neutralization and flocculation. 
FerriTabs are non-toxic and completly water soluble. They contain no caustic or corrosive chemicals, alum, diatomaceous earth, or any other harmful chemicals that would alter the pH or chlorine level of the water. They will not harm any of your swimmers either!
How Do I use FerriTabs?
Make sure pool water has a pH of 7.2-7.6, and contains a measurable amount of chlorine by adjusting with chemicals as necessary. Chlorine will not effect the Ferri-Iron Tab action on Iron or Manganese. Make sure the pool filter is in good operating condition and has been turned on.
Dose: 1 tablet/3,000 gal.
EASY-TO-FOLLOW STEPS:
(Based on an average 20,000 gal. in-ground pool)
Step 1: Drop 3 tablets into the skimmer basket. *If no skimmer basket is present directly into the pool is fine
Step 2: Circulate the Pool for 24 Hours
Step 3: Backwash or clean the filter. If the backwash water looks iron filled then repeat step 1 & 2 until backwash water runs clear and pool quality improves
In pools that have severe problems, repeat dosage and back-wash procedures after 24-48 hours.
In order to prevent a recurrence, when adding make up water and 1 tablet for each 3000 gallons or less of fresh water, circulate 24 hours & backwash.
***Note: This product DOES NOT remove iron staining from pool surfaces. This product removes iron from in the water itself. You must pre-treat with an ascorbic acid based stain remover, and then treat with FerriTabs to completely remove staining and iron from your pool.***