The 2510 adapter should only make contact on top of the tank plus 1/4 turn. Do not overtighten it. It was probably put on by a gorilla many years ago. no thread sealant!, only dow corning 7 release compound should be used. http://www.pentairwatertreatment.com...Components.pdf
O-rings are ordered individually.
10x54 tank should have approximately 15-18" of freeboard depending on media, gravel underbedding depth etc. your tank is slightly over filled.
For iron removal, and general applications, 10 PPM of chlorine is normal and not a problem, the carbon tank will easily remove the residual chlorine. If your iron levels are excessive, higher chlorine levels may be needed.