Steve, it's a Pac-Fab Nautilus, I don't know the model # but I believe that it has ~ 36 sq ft of filtering area. On a similarly sized Hayward it should take 4.5 lbs of DE fully cleaned and 3.5 after backwashing. If I remember correctly Pac Fab tells people to put in 5 lbs clean, which would thereby require 4 lbs after backwashing. Give 5 lbs a try, if I see a Pac Fab in the near future I'll get the model and recomended DE off the lable (if it's not as faded as yours )