I don't think the tap water will kill your pump, though I am not sure of your local water quality.
I am having trouble finding you pump(maybe I am brain dead
). Could you give some more information about it? All the Uno pumps I am finding are much too big to be what you intend.
Your water being soft(which is good, less scaling) doesn't tell me a whole lot other than you likely live in an area where the ground (your bedrock specifically) is low in calcium and magnesium.
Their are two reasons for wanting to use distiled water.
1. It keeps biological activity to a minimum.
2. It is better at fighting corrosion than tap water, generally.
How much is the distilled water per liter in South Africa?
Where I am four liters can be had commonly for $1 USD (~8 Rand).
Unless you need more than 20 liters I would stick with distilled water.
Painting the outside of your tubing, assuming it was clear to begin with, should slow down bio-growth.