Spirits of Salts is HCL Hydrochloric Acid which is wonderful for removing scale and calcite build up...
AHA! Also known here by the archaic name muriatic acid, commonly used by boat owners worldwide to dissolve scale buildup in hoses. Sold in the UK as "brick cleaner." A 12-15% dilute of whatever the dilute in the bottle (it varies with brand). For future reference, buildup can be prevented with a weekly cupful of distilled white vinegar flushed through, followed in an hour by at least a quart of clean FRESH water.
The toilet can only ingest sea life via a line through which water is being PULLED--i.e. the toilet's flush water INTAKE hose. Even when no sea life has been pulled in, sea water is full of micro-organisms that when left to sit in the toilet's intake line and pump while the boat sits unused even for a day or two die and decay...causing the first flush upon return to knock you off your feet! However, that odor only lasts till the stagnant water has been rinsed out by new. That you're seeing black flecks in it means that the toilet definitely has pulled something into the intake plumbing...So you need to follow the instructions I gave Gmarr above to flush it all out.
Chris, I think "overflow hose" hose refers to the toilet/tank overboard discharge, but to be sure...
Phil, does that hose come off a fitting the top of the tank or a fitting at the bottom? If at the top and a smaller diameter than the toilet-to-tank hose, it's not an overflow, it's the tank vent line. Only AIR should ever pass through through it!
...my wife who loves the boat having had enough!!!!!!!!!
I know exactly how she feels! A similar problem on the first boat big enough to have a toilet and holding tank that my late husband and I bought 33 years ago was so bad that either the odor had to go or the boat had to go, and I liked the boat. Holding tanks were so new in the '80s that the only thing anyone in the marine industry had to offer was "boat toilets stink holding tanks stink worse." So I had to go outside the marine industry to sewage treatment experts to get any helpful information. Learning how to solve it and then helping our dockmates who had the same problem became the accidental start of a whole new career. Though most people don't consider a move from 20 years in advertising to sewage management to be a major career leap.
--Peggie