NtLabs Oxygen Test Kit for Ponds Oxygen is essential for the fish, plants and filter bacteria. The concentration of dissolved oxygen in the water can drop to dangerous levels causing hypoxia stress. The most accurate Oxygen test kit on the market! For a healthy pond and fish, oxygen (O2) levels must be high. Cold water carries a lot more O2 so this is less of a concern in winter months. Test regularly in summer months when the temperature rises and O2 levels drop, especially in humid conditions. A liquid test kit. Includes Contains 25 Tests