Knowing what not to put in your fish pond can be just as important as knowing how to set one up properly. Pond water isn’t just a feature in your garden, it’s the entire environment your fish live in. Anything that enters it and shouldn’t be there can cause serious harm, sometimes within hours. This piece covers the most common mistakes pond owners make, why they happen, and what to do instead.
Don’t Try to Clean It Yourself
No matter how dirty or green your pond gets, don’t attempt to clean it yourself unless you are using professional pond cleaners specifically designed for pond water. Using household disinfectant or chlorine in your pond will not work and will harm your fish.
Tap water contains chlorine too, which is worth remembering if you’re topping up after evaporation in summer or refilling after a clean. Always treat tap water before it goes into a pond with fish in it, or use a proper dechlorinator. If you’re unsure about the right products to use, our pond maintenance packages cover water quality care as part of a structured annual plan.
Why I Know This Matters
Nearly 20 years ago, my parents had just moved garden centres and needed to spruce the place up to increase trade. They had a few fish tanks, so I asked my dad if I could look after the aquatics section and make something of it. He said yes, told me not to spend much, and to make sure I brought more in.
We got new tanks in, built a decking area, brought in new stock. But it still looked a little tatty. We had a tonne of paint available, so I proceeded to paint the entire aquatics section blue. It’s actually the same blue we use in our company logo today.
It took weeks to complete, painting between serving customers and covering the shop. One day, working over the top of the goldfish tanks, two droplets of paint fell into one of them. Within the hour, four or five goldfish had died.
It was mortifying.
But it taught me a lesson I haven’t forgotten. Water in a pond is the fish’s environment in the same way air is ours. Anything that enters it and shouldn’t be there is poison. We have to treat pond water the way we’d want others to treat the air we breathe.
Common Mistakes That Harm Pond Water Quality
Beyond chemicals and paint, there are several everyday errors that quietly upset the balance of a fish pond over time.
Overfeeding your fish
Uneaten fish food breaks down in the water and adds excess nutrients and phosphorus, which fuels algae growth and reduces oxygen levels. Feed little and remove any uneaten food after a few minutes. In cold weather and through winter, fish metabolism slows significantly and they need far less feeding, if any at all.
Letting debris build up
Leaves, garden waste, and organic matter that settle on the pond surface and sink will decompose and release nutrients into the water. In autumn particularly, a fine net over the pond surface helps keep debris out and reduces the maintenance needed come spring.
Filling or topping up with untreated tap water
Tap water contains chlorine and in some areas relatively high levels of phosphorus, both of which can harm fish and upset the pond’s natural balance. Always treat tap water before it goes in, or ideally use collected rainwater where possible. For a full breakdown of what to test and when, see our guide on how to test pond water.
Removing too much water at once
If you’re carrying out maintenance, avoid removing large volumes of pond water in one go. The beneficial bacteria that keep the water healthy live throughout the pond, and removing too much at once can disrupt the balance it’s taken weeks or months to establish.
Blocking access to the surface
Fish and pond wildlife need access to the pond surface for oxygen. Avoid letting algae, debris, or even large pond plants cover the entire surface area. Oxygenating plants are helpful for oxygen levels but the surface itself needs to stay largely clear, particularly in summer when oxygen levels in warm water naturally drop.
Ignoring ice in winter
Never break ice on a pond by force. The shockwave can harm or kill fish. Instead, place a pan of hot water on the surface to melt a hole gently, which allows harmful gases to escape and lets oxygen in. A pond heater or de-icer is the most reliable solution if your pond freezes regularly.
The Principle Behind All of It
Every mistake on the list above comes back to the same thing: pond water is a living environment, not just a body of water. Fish, oxygenating plants, small creatures, and beneficial bacteria all depend on it staying in balance. Once that balance is upset, it can take a long time to recover, and in the worst cases, fish don’t survive long enough for it to do so.
If you’re ever in doubt about whether something is safe to put in your pond or add to the water, ask before you act. Big lessons come from painful experiences, and some of them don’t need to be repeated.
If you need guidance on your pond, get in touch and we’ll do our best to help. If you’re considering a professionally designed and built fish pond, we’d be happy to talk through what’s possible for your garden.
Frequently Asked Questions
Can I use tap water to fill or top up my fish pond?
You can, but it needs to be treated first. Tap water contains chlorine, which is harmful to fish and disrupts the beneficial bacteria that keep pond water healthy. Use a dechlorinator or chlorine neutraliser before adding tap water to the pond. In summer, evaporation can lower water levels noticeably, so topping up is often needed, just make sure it’s treated water going in.
Why do my fish keep coming to the surface?
Fish coming to the surface regularly is usually a sign that oxygen levels in the pond are low. This is most common in summer when warm water holds less dissolved oxygen, and can be made worse by algae covering the surface or a pump that isn’t circulating water effectively. Check your pump is running correctly, clear any surface coverage, and consider adding oxygenating plants to help maintain oxygen levels throughout the day.
How much maintenance does a fish pond actually need?
It depends on the size of the pond, the number of fish, and how well it was set up initially. As a general guide, you’ll want to remove debris regularly, check the pump and filter are running, monitor water quality, and carry out a more thorough clean in spring before fish become more active. Avoid doing everything at once. Spreading maintenance tasks out keeps disruption to the pond’s balance to a minimum.
Is it safe to put wildlife pond plants in a fish pond?
Most aquatic and oxygenating plants are fine and actively beneficial in a fish pond. They help produce oxygen, absorb excess nutrients, provide shade in summer, and give fish somewhere to hide from predators. The main thing to avoid is introducing plants that have come from another pond without checking them first, as they can carry pests, disease, or invasive species. Always source plants from a reputable supplier and rinse them carefully before they go in.