I would have to look up my research to explain why GH and KH can be vastly different, but suffice it to say, they can.
If this were my problem, I would first get the tank water where I wanted it by mixing "pure" water with the tap water (pure being distilled, RO or rainwater, the latter is ideal as it will be acidic). Once this is achieved, do smaller-volume water changes so there is not such a drastic shift. Once an aquarium has biologically stabilized, it tends to "stay put" and smaller water changes will not affect it as much.
On the angels and Discus
, this is not advisable. The feeding issue is one important aspect, but another is temperature. Discus
must be kept warm, 82F is minimum. Angelfish will be better at lower temperatures, around 77-78F. Wild angels are different, they also need warmth, but not the usual commercially-available fish. [This difference does not hold for Discus
, unfortunately.] Fish should usually be maintained in the lower half of their preferred range; the higher the temperature the harder they must work and in an aquarium this can have repercussions.