gH/kH in a nut shell. GH: calcium and magnesium disolved in the water. Typically high in the lakes that african cichlids come from. Typically low in the amazon.
KH: Ammount of carbonates in the water. Usually this is your buffering capacity, but sometimes there are other buffers if the water.
KH is what gives your tank pH stability, and is consumed via natural processes. If the KH is not replenished, ie via water changes, both the pH and KH will begin to drop. When the KH reaches 0, then there is nothing left that can be used by the biofilter and the good bacteria will begin to die off. Usually at that point the pH of the tank is at ~6.0.
This is what the term "pH crash" refers to, although a better term would be "KH crash". However, since most people do not monitor their KH, the dropping pH is all they see.
So long as you are keeping up with regular weekly water changes of 25% or more, there is no danger of a 3dKH dropping to 0.
In fact, the only time you should even worry about a KH of 3 is if you decide to add a lot of plants and inject CO2. 3dKH is "borderline" and some people who inject buffer it up and some do not.
If it concerns you, the best *temporary* course of action is to use baking soda to increase the KH/pH. Adding chemicals to the tank increases the TDS (Total Dissolved Solids) of the water and should be avoided when at all possible. That includes the stuff the LFS was trying to sell you.
For long term or "permanent" buffering, a handful of crushed coral and/or aragonite put in a sock or nylon stocking and placed in your filter will provide a permanent increase. Coral/aragonite takes a while to dissolve at pH levels over 7.0, so it may take a week or two for anything to happen. Baking soda will serve as a buffer in the meantime, and if you monitor the KH and note when it begins to rise on its own above that which has been set by the baking soda, then the soda is no longer needed.