It is extremely hard to nearly impossible to get stable high levels of both calcium and alkalinity. There is saturation and supersaturation. High levels of both is supersaturation where some thing must precipitate under most conditions. Usually people who super saturate will do so with calcium and in doing so run they must run lower alkalinity. You have the opposite situation, you are supersaturated with carbonates, and therefore low in calcium. Kent Turbo Calcium is not made to raise your calcium levels specifically, it is supposed to be used after you have raised the calcium levels up with Kalkwasser. I would suspect the Kent product mainly increases calcium carbonate levels not calcium levels. That would explain there advising you to "for maximum results use Kalkwasser first, then raise calcium with this product." That realkly makes little since because Kalkwasser is adding calcium. It is hard to tell though with out knowing their specific ingredients though. I would speculate that it is just another product that supplies more calcium carbonates than calcium. Other examples of products such as this are products that are made of aragonite. Aragonite typically is higher in calcium carbonate than it is in calcium.
RO water is high in CO2 and low in oxygen, it should be well aerated before use. There are also many problems associated with tap water if it was softened by a water treatment plant, they typically soften water with lime and soda ash.
There are no commercial salt preparations that normally mix up to produce water with either calcium that low or alkalinity that high.
In a new tank with out stoney corals I would not worry about calcium below 350 or 375 ppm, and all mixes supplied commercially should mix up to provide levels over that amount. It is possible that you got a bad bag from a poorly mixed batch of salt mix. But I would use some well aerated RO water and mix up just a small batch (1 gallon) and test it for levels of calcium and alkalinity before adding it to the tank. Aerate the RO water at least overnight and mix the salt in the water with a power head overnight. If that water tests well, then you have water changes to do. That is a safer method then lowering the alkalinity followed by then raising the calcium.
Please consider discussing any additives to your tank before using them. It is usually best to try to figure out the cause of the problem before correcting it, and besides there are a lot of products out there that are not labeled very well. It is very easy to use the wrong products with out researching and asking question first. Manufactures do thing like labeling things as buffers when they are not and other things such as that. Nearly all the manufacturers misrepresent/mislabel calcium and calcium carbonate products constantly.
