*Product May Be Subject To Extended Lead Times. The UltraZap Gravity Filter 25 Litre is a compact yet powerful pond biofilter designed to maintain a clean, healthy aquatic environment. Utilizing a natural biological process, this filter supports colonies of nitrifying bacteria that play a vital role in water quality. These beneficial microbes convert harmful ammonia and nitrites—byproducts of fish waste and organic decay—into far less toxic nitrates, keeping your pond ecosystem balanced and safe for aquatic life. To keep these bacteria alive and active, continuous oxygen flow is essential. That’s why it’s crucial to run your pond pump 24/7—not only to keep your waterfalls flowing and water circulating, but also to ensure adequate aeration. Without this, oxygen levels will drop, causing the bacteria to die off and compromising your pond’s health. The UltraZap Gravity Filter is a reliable, low-maintenance solution that works in harmony with your pump to support crystal-clear water and a thriving pond environment. The Cap for this filter is 4000 L and 250 Bio Balls