An attachment assembly for attaching a fluid container to a fluid warmer of a system for extracorporeal blood treatment is disclosed. The assembly includes a hook-shaped fixture located on the fluid warmer, a pin-shaped fixture located on the fluid warmer, and receiving members located on the fluid container to receive the hook-shaped fixture and the pin shaped fixture located on the fluid warmer. A fluid warmer containing a hook-shaped fixture located on the fluid warmer and a pin-shaped fixture located on the fluid warmer of a fluid bag attachment assembly is also disclosed. Tubing guides present on the fluid warmer in conjunction with tubes of differing lengths attached to the fluid bag reduce the occurrence of usage errors during insertion of the fluid bag into the fluid warmer.