Struvites are a significant constituent of kidney stones and form in waste tanks when the pH of the solution is basic. Because sea water had a pH of 8+, sea water flush toilets are more prone to struvite formation (and other precipitates) than fresh water. The cure is prevention, by keeping the pH of the tank acidic, which can be done with vinegar, or injection of a formaldehyde solution like San-X, which has the added benefit of killing e coli, should you care. And just FYI, but formaldehyde breaks down quickly in the environment, has the same toxicity as bleach, so don't drink bleach or San-X, and don't stick your nose into either bottle.
Tom, not sure if your using sea water flush or not, but if you're concerned about precipitates, dump a couple of gallons of white vinegar into the tank. As long as the pH is acidic, struvites won't form and if they have will slowly dissolve in a week acid solution.