To buffer, you need to increase the KH (Karbonathärte), which is the carbonates (CO[sub]3[/sub][sup]2-[/sup] and HCO[sub]3[/sub][sup]-[/sup]). As I mentioned earlier, this can be done with crushed shells and reef bones (dead rock which was formed by dead corals), both of which contain calcium carbonate. Since you are not interested in increasing the hardness, it would be enough to use only a small quantity of either or both (the difference being that you'd need to use a larger quantity to increase the GH). Note that many aquarium test kits refer to "total hardness" (KH + GH) as GH (which is quite misleading), while water suppliers and most sources refer only to metal ions as GH. In other words, if your KH increases, the GH reading which your test kit gives you is likely to increase by exactly the same or slightly higher amount. It would be almost impossible to affect only the KH without having any effect on the GH, so do not be surprised if you see the GH increase by a fraction or even a few degrees (depending on how much you aim to increase the KH by).
As for how many much to add? I would start with 2-3 whole mussel shells or 300-500 g of reef bones per 60 litres and experiment from there (as opposed to GH increases, for which I would be recommending starting with 6-8 mussel shells or 1 kg of live rock per 60 litres). With time, the deterioration of the shells will become visible and they will need to be added to or replaced, but rock doesn't usually exhibit much change over the years. If you are placing the shells in the filter, I recommend that you make sure they can be easily removed, in case you need to adjust the rate at which they buffer the water. If you're going to be placing them straight onto the substrate, I recommend that you start with whole ones and crush them only once you know how many you want to keep in the long term (so it's easier to remove any potential excess). Crushed coral (which is often sold for marine aquarium use) tends to be more potent than either shells or rock because it is more porous, and is usually used in stockings inside the filter.
