Good acid for cleaning marine toilets Problem:
On my boat I have a marine toilet. It is operated with salt water.
The problem is, on the interior of the outflow tubing it accumulates some sort of hard grime. I am looking for a good way to dissolve this grime as it is impossible to replace the tubing.
From experience it is known that citric acid work - but not very well. Also it leaves a &quot;sandy&quot; residue that is difficult to flush.
Does anybody have an idea of what this &quot;grime&quot; consists of - and thus how best to dissolve it?
Materials:
On hand I have:

citric acid
56% phosphoric acid
23% HCl

Materials:

tubing: PVC
toilet itself: porcelain
pump: ABS or polypropylene with selected parts in acetal resin, with 316 passivated stainless steel fastenings, brass weights and neoprene seals and gaskets
valve: grey, harder plastic

I know this quite vague but am hoping somebody can help anyway.
A cheaper and easier to find solution than EDTA (permeakra's answer) is citric acid. Citric acid reacts with metal carbonates (bicarbonates, phosphates, chlorides, etc.) to produce the soluble citrates. The citrate ion keeps the metals in solution by chelation, the same process that EDTA (ethylene diamine tetra acetate) does. 

Citric acid can be purchased in the grocery store or convenience store as one of the principle components for any sour powdered drink mix (like Tang).

You could also use any substance or cleaner designed to remove hard water deposits or lime scale, such as CLR, which contains lactic and gluconic acids as chelating agents.