He means 0.5 like these
http://www.reefsupplies.ca/online-st...ectrapure.html
And
http://www.reefsupplies.ca/online-st...bon-block.html
0.5 prolong the life of the DI resin. We don't even carry 5 micron anymore as 80% of people are buying 1m and the rest are buying 0.5m
Ray*