Potassium Metabisulphite is a very cheap chemical. It's widely used as a preservative and cleaner in winemaking, brewing and other food preparation, and is available in convenient tablet form. It forms Sulphurous acid in solution, the same acid as in Fixing baths, hence the same pungent smell. This also makes it totally compatible with normal acid fixing baths(posted 8476 days ago)Pinholing is caused by carryover from the developer outgassing within the emulsion. The main culprits are developers with a high carbonate content, and higher than normal processing temperatures. The carbonate breaks down to give off carbon dioxide in any acid solution.
The 'cure' would be to use a two bath stop, the first plain water, and the second a weak acid bath to prolong the life of the fixer. This kind of defeats the secondary purpose of the stop bath, which is to rapidly arrest the development.