Salinda, if FC + CC doesen't = TC, there is a testing problem, either the reagents are no good or you are reading the results wrong (believe me, I know how hard it can be to compare the colormetric tests at times, judging the variations between the 'given' values vs. your result). If you think that the reagent might be bad, replace it.
I suspect that the alk is dropping due to airation (do the returns point up?, do you have a 'water feature' that would make the water bubble?), as your pH is 7.0. This is consistant with the alk reduction method advocated here (low pH and some airation will result in the alk steadilly dropping). Also, how low can you test with your kit, if 7.0 is the bottom, it's very possible that the pH is actually lower than 7.0. Go ahead and add the baking soda, it'll raise your alk and also drag the pH up (yes, it's fine for SWCG pools).
I hope this helps, if you have any further questions or need clarification - don't hesitate to ask. - Waste
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