You are going to have to remove the bumper to gain good access, I don't think you can reach the pumps if you remove the wheel arch liner, as if I remember correctly they are on the front side of the tank.
To combat the slime I use a food safe sanitiser, I put just a splash ( about a tablet spoon full if you want actual amount) in to my 5ltr top up bottle, when refilling the washer bottle , but only in the first fill of the top up bottle.
This is the one I use only because it came free from work, were we used it to sanitise the condensate drains on ventilation equipment.
