Put in a few drops of iodine and use distilled water not tap water. Iodine is an effective biocide and requires a tiny amount to be effective. You don't need more than 10 drops per liter. That is overkill for distilled water. You'd probably be safe with 2-5 drops. You could use Copper(II) Sulfate drops and Iodine Drops.
Some people use a 10% anti-freeze mixture. But anti-freeze is not as efficient as water. Others use rubbing alcohol. I'd think 5% to 10% ABV would work. As with anti-freeze it reduces efficiency. While the alcohol should be as effective. I don't think the anti-freeze will be as effective as Iodine.
If using clear tubing you could place some ultraviolet LED bulbs by it.