Give your tank shock therapy
1/4 cup household bleach for every 15 gallons of water. Shocking means leaving the bleach in the system for several hours and then draining and refilling with fresh water. With regular use, shock treatment is hardly, if ever, needed. If you are not traveling on a frequent basis and the water sits for months, then I would do a shock treatment before your next trip. If you travel every couple of weeks or so, then you might not worry about bleaching since your waters not stagnant.