The only essential ingredient in a fixer is thiosulphate. All the rest is secondary, being preservative (primarily) and pH adjusting, and perhaps hardening. Rapid fixers are ammonium thiosulphate. If they are acidic they smell of SO2. If they are alkaline they smell of ammonia. If they are close to neutral (i.e. between acidic and alkaline) they have minimal smell. If you use materials from mainstream companies you don't need hardening, so use something like Kodak Flexicolor Fixer (pH=6.5) which is cheap, rapid and not very smelly. It's sold for color procesing but is totally functional for b+w fil and paper.
Originally Posted by Ian Grant