As the days shorten and water temperatures begin to drop, your pond enters a critical transition phase. While spring gets all the attention for fish health challenges, autumn is equally important. As water temperatures fall below 15°C, a koi’s immune system naturally slows down—yet many parasites and bacteria remain active.

Catching health issues now ensures your koi enter the cold winter months strong, clean, and healthy. Here is the N-Koi guide to diagnosing common autumn issues and taking action before winter sets in.

The Early Warning Signs (What to Look For)

Before physical symptoms appear, your koi’s behaviour will tell you if something is wrong. Spend 10 minutes observing your pond during feeding or quiet periods.

• Flashing and Scraping: Fish flicking themselves against pond walls or bottom drains. This is a classic reaction to skin or gill irritation (usually parasites).
• Lethargy or Isolation: Sitting away from the main shoal, remaining near the surface, or resting at the bottom with fins clamped tight to the body.
• Gasping at the Surface: Hanging near waterfall returns or air stones, indicating gill irritation, poor oxygenation, or elevated nitrite.
• Reddening or Excess Mucus: Pinkish patches on the skin, clear vein visibility in fins, or a cloudy grey coating (excess slime coat produced to fight off parasites).

Four Steps to Protect Your Pond Before Winter

Step 1: Test Your Water Quality Water quality is always the foundation of fish health. Ammonia and nitrite irritate gills and weaken immunity, making koi vulnerable to secondary infections. Keep your water parameter thresholds locked in:

• Ammonia: 0.0 mg/L
• Nitrite: 0.0 mg/L
• pH: Stable between 7.5 – 8.5

Step 2: Do a Microscope Diagnostic (Skin Scrape) Never dump blanket medications into your pond without knowing what you are treating. Taking a quick skin scrape from an affected fish under a microscope identifies the exact parasite so you can use the correct targeted treatment.

Step 3: Clear Out Debris Falling leaves and organic sludge sitting on the pond floor break down into harmful gases and feed unwanted bacteria. Net out leaves daily and keep your bottom drains and filter bays clean.

Step 4: Support Immunity with Proper Nutrition As water drops below 12°C, switch off high-protein diets and introduce an easily digestible Wheatgerm food or high-quality Premium Blend to support digestion without straining the water quality.

Visit N-Koi for Health Advice & Treatments

Unsure about a symptom or need help selecting the right water treatment? Whether you drop by to pick up water testing kits, treatments, or just want to chat through a health concern, Neil and the team at N-Koi are here to help you keep your pond in peak condition all year round.