Plenty of people keep successful tanks without those two kits but they really do help with pinpointing your water's specs.
With proper waterchanges and good habits, most of the quality brands of salt mixes have ratios in line with what you will need. So dont worry too much about it. Grab them if you can down the road but not absolutley manditory.
How long have you had the carbon in the water? And when really looking close, does the water look like dust in the water, as in small flakes? or just so fine you cant make it out?
More of a curiosity than actual scientific analysis but strange things can react with one another. Switching salts, foods, or water supplies for that matter can do funky things. Very rare for a salt mix to be so far off as to cause a chem reaction.
Some places have tap that acts up from time to time. Back to the carbon, how long has it been in?