Avoid chemicals that adjust pH. They often contain things you don't really need to add to a tank. The only thing I would even consider is remineralisation salts as all they add is minerals found in naturally hard water and you add only enough to get the hardness you want.
If the shells don't work you could always try changing the substrate to coral sand. This is mainly used by people keeping Rift Lake cichlids which need very hard alkaline water, but that is in addition to using special salts to get the water hard and alkaline. But with your very soft water using coral sand would probably give you slightly raised levels. There would be more of it than shells and the smaller particles would dissolve faster. Calcium carbonate (shells, coral, limestone etc) is officially insoluble in water but it does dissolve very slowly.
This is the hardness testing pack I had;
this is Nutrafin. Whichever you get, they work differently from pH, ammonia etc. Instead of adding x drops, waiting a few minutes then comparing the colour to a chart, they work by adding one drop at a time until the liquid changes colour. With API, the number of drops = hardness in german degrees and that can be converted to ppm by multiplying by 17.9. With Nutrafin, the number of drops x 10 = hardness in ppm (or mg/l, which is the same thing)