Setting Up a Small Goldfish Pond: Water Prep, Filtration, and Algae Management

Before Adding Fish to a New Pond

1. Dechlorinate: If using tap water, add a water conditioner (e.g., Seachem Prime, API Pond Stress Coat) *during* filling.
2. Add salt (optional): 0.1% pond salt (1 lb per 100 gallons) reduces stress and supports slime coat. Not needed if you have plants.
3. Seed beneficial bacteria: Use a liquid starter culture (Microbe-Lift, API) to jump-start the nitrogen cycle.
4. Aerate well: Run the pump/fountain 24/7 for 48 hours before adding fish to oxygenate and off-gas CO2.

Filtration: Yes, You Need It

Even a 30–40 gallon pond needs a pump and filter. A small submersible filter with sponge and bio-media, or a DIY bucket filter, prevents stagnation and processes waste. Frequent water changes alone aren't enough—ammonia spikes between changes.

Algae: Some Is Good

• Green film on walls = healthy biofilm. Leave it.
• Green water (pea soup) = excess nutrients. Add plants, reduce feeding, ensure filter works.
• String algae = manually remove; add barley extract or spot-treat with hydrogen peroxide.

Plan for Growth

Six goldfish will quickly outgrow 40 gallons. They can reach 12+ inches. Aim for 100+ gallons long-term, or rehome extras. A larger pond is more stable and easier to maintain.