The reason for not using bicarb with fish is that if KH is low, GH will also be low, so the water is suitable only for soft water fish. But soft water fish have not evolved to cope with a lot of sodium in the water. It's the same reason why water from a water softener which uses salt should not be used with fish - that type of water softener swaps the hardness minerals for sodium. Gradually adding bicarb will not overturn thousands of years of evolution.
Sodium also needs to be in balance with potassium, and if there's too much sodium compared to potassium - that is, they are out of balance - it is harmful for fish.
There are some fish which are OK with bicarb - Rift Lake cichlids. The salts made to increase hardness for these fish does contain sodium, but the water in the Rift Lakes is unlike any other on Earth.
I would treat the people at Tropco the same as other people selling things in the fish trade - with a huge pinch of salt. They are right that KH needs to be higher than yours during cycling, but not once there are fish in the tank.
Shells - these are made of calcium carbonate like coral and limestone. But they must be clean and not coated with lacquer or any other type of coating. But why bother trying to increase GH, KH and pH (which is what coral, shells and limestone will do) when it is much easier to keep soft water fish which will be quite happy at your low levels?
they have some cool carp in a pond and lots of chickens, I'm sure I saw an emu in the distance when I went there once!
