Will muriatic acid remove hard water deposits from a plastic tank used to mix salt water for aquarium use?

HCL or CLR are okYes, however it may not be the best choice. Muratic acid (HCL) is Hydrocloric acid and used in Swimming Pools and labs to adjust pH. It also generates free chlorine which may be bad for some fish and your containers.

Not all plastic tanks are acid safe so check the codes for HDPEhttp://www3.sympatico.ca/n.rieck/docs/plastic_codes.HTML

Then TEST a small spot.

Better yet a hard scale or lime remover like CLR may work better and dissolve faster.

A strong Vinegar or a weak Sulphuric Acid solution would also be better than a HCL solution for fish.

Most fish are very pH sensitive so rinse very well...