The official document for Drinking Water Quality analysis can be found on the PUB website.
Note that the above document does not contain any information on potassium (K).
Potassium Content in Singapore tap water
I managed to find another source (dated 2002) on potassium content in NEWater: http://uwatech.com/wp-content/uploads/2015/11/newater-study-report.pdf. In page 16, the potassium levels are stated as:
Range (Potassium): 0.504 to 3.07 mg/L
Mean (Potassium): 1.08 mg/L
Sodium Content in Singapore water
Range (Sodium): 3.16 to 42.1 mg/L
Mean (Sodium): 13.35 mg/L
Potassium in Planted Tanks
For aquarium plants, “potassium content should be at least equal to sodium content or higher.” (http://aquayer.com/index.php?page=kaliy-v-akvariume&lang=en)
Usually, the solution is to dose potassium fertilizers (for example potassium sulphate).
In addition, many sources recommend a level of around 20 ppm of Potassium for planted tanks (see for instance this site). That is 20 times the amount found in tap water! Other sources push it further, there are sites that recommend up to 50 ppm of potassium.