The rule of thumb is that you make a lot of solutions and see which one is the easiest to handle and that the customer will pay for.
There are way too many variables in this problem to even attempt a solution without some details. Even the one hypothetical you gave shows enormous variability. Let's suppose you do use sodium carbonate - what temperatures will you expect the solution to be exposed to? Are you shipping any to Norway? Egypt? In sealed shipping containers exposed to sunlight? The solubility of sodium carbonate is 71 g/L at 0°C, over 450 g/L at 100°C, so unless you are using a large excess of water, you can expect precipitation from the solution at some point. Not to mention the exchange of carbon dioxide with the air, which will also change the concentration of carbonate in solution - carbonates are notoriously unstable in solution due to carbon dioxide transfer with the air. Then you have the shipping issues - will shipping the excess weight of water be worth the effort and expense, even though handling liquids is much faster? What about spill clean-ups? End-user handling? It's a lot easier to lift 15 pounds of sodium carbonate solid to throw into the tank than it is to lift the almost 100 liters of water it would take to dissolve it all at 0°C.
Formulations is a science all its own, and chemistry is only a small part of the solution. Even for an answer to just the chemistry part, knowing what chemicals you are handling and how they will be handled is critical to even begin looking for an answer.