56 · Frequently Asked Questions

The 20 questions pond owners ask most, answered straight.

Basics

How deep should a pond be?

At least 60 cm for fish, 80–120 cm for koi. Wildlife-only ponds can be shallower (30–45 cm) with a beach entry.

How big should my first pond be?

Bigger than you think. Minimum 1,000 L for a few goldfish; 5,000 L+ for koi. Large ponds are easier to keep stable.

Do I need a liner?

Almost always yes. Only skip the liner if you have heavy clay soil (30%+ clay, 30 cm+ depth) and accept some seasonal water loss.

Can I use an old bathtub or container?

Yes — a great way to start. Seal any drain holes, add gravel and plants, and you have a container pond in an afternoon.

Water quality

Why is my water green?

Algae bloom from excess nutrients (overfeeding, too many fish, runoff) and too much sun. Add shade plants, reduce feeding, and add oxygenators.

How do I clear green water without chemicals?

Reduce light (shade plants), reduce nutrients (less food, more plants), and add a small pump for circulation. It may take 2–4 weeks.

Do I need to test my water?

Monthly testing (pH, ammonia, nitrite, nitrate) catches problems early. A simple test strip kit costs under £10.

What is KH and why does it matter?

KH (carbonate hardness) buffers pH. Low KH (<80 ppm) means pH can swing dangerously. Add crushed coral or limestone to raise it.

Fish & wildlife

When can I add fish?

After the pond has circulated for at least 2 weeks and ammonia/nitrite read zero. Add just a few at first.

My fish are floating upside down!

Likely gas bubble disease (supersaturated oxygen) or ammonia poisoning. Test water immediately; add aeration; do a partial water change.

How do I keep herons away?

Netting, motion-activated scarers, deep water with hiding places, and overhead wires all help. No single method is 100%.

Maintenance

How often should I clean my filter?

Rinse mechanical media in pond water (never tap) every 2–4 weeks. Leave biological media alone unless flow drops.

When should I do a water change?

10–20% every month in spring/summer. Always dechlorinate new water. In a well-planted pond, water changes are less critical.

Should I drain and clean my pond each year?

No. Draining removes the beneficial biofilm and stresses fish. Partial cleaning and sludge removal is sufficient.

Cost & effort

How much does a pond cost?

A basic wildlife pond: £200–500 DIY. A koi pond with filtration: £2,000–10,000+. A swim pond: £5,000–20,000+.

How much time does it take?

15–30 minutes per week in summer; nearly nothing in winter. A well-planted pond is self-regulating.

Will a pond add value to my house?

A well-maintained pond adds appeal. A neglected, algae-filled pond detracts. Buyers see ponds as either a feature or a liability.