Fish are usually more concerned about the GH. They need to calcium and magnesium in the water.
As far as I know fish do not use the carbonates or bicarbonates. However, these do buffer the pH, and that is important. With very low carbonates the pH can fluctuate quite a bit, and some fish do not do well with that.
Very roughly, if you can keep the GH in the right range for the fish, then make the KH about equal to the GH, then the water will probably be pretty close to the right pH for the fish. In terms of importance I adjust my tank water this way:
1) Get the GH right.
2) Get the KH fairly close.
3) Let the pH do what it will...
3a) For soft water fish filter the water through peat moss to add organic acids.
3b) For hard water fish the higher KH will usually set the pH in the right range. Adjust the KH if needed. Do not monkey with the pH directly.