The fungicide has ten charges when new, after which a refill must be placed in it to continue using. The refill gives the fungicide spray 10 more charges, with 10 being the maximum amount of charges that can be held at one time. It is purchasable from any Slayer Master for 300 coins, with refills costing 10 coins more. Somewhat ironically, fungicide spray refills are a common drop from Zygomites despite fungicide spray being the item used to kill them. However, you need a fungicide spray in the first place to kill them so getting a refill saves the inventory space one would need to carry the extra sprays.