Your essential guide to seasonal shifts in pond and aquarium care

As summer fades and September rolls in, fishkeepers across the UK are met with subtle but important changes in both pond and aquarium environments. While the days may still be warm, nights begin to cool, and your fishkeeping routine should adapt accordingly.

Whether you’re caring for an indoor aquarium or a thriving outdoor pond, this guide will walk you through what’s changing this month — and how to prepare your fish and systems for the season ahead.

1. Falling temperatures = behavioural shifts in fish

What’s changing?

By mid-September, average UK temperatures begin to drop, particularly overnight. This will impact fish activity, metabolism, and feeding habits.

What You Should Do:

  • Monitor water temperature — Use a pond thermometer to track dips.
  • Reduce feeding as temperatures fall. Fish metabolism slows down, and uneaten food can pollute water.
  • Start transitioning to wheatgerm-based autumn food for pond fish — it’s easier to digest in cooler conditions.

Tip: Stop feeding entirely when water temperatures fall to 8°C or below. Fish won't digest meals properly, and it can lead to health issues.

Falling temperatures = behavioural shifts in fish

2. Water quality

What’s changing?

With decreased biological activity in cooler water and leaves beginning to fall, water parameters can shift  — particularly in ponds. Like indoor aquariums, ponds with few plants may also suffer from more stable but overlooked issues like gradual nitrate build-up.

What you should do:

  • Test your water for ammonia, nitrite, nitrate, KH and pH levels.
  • Clean out pond filters and skimmers before leaf fall increases debris.
  • Top up evaporated water with dechlorinated tap water, especially in tanks with open lids.

Best Practice: Use a dedicated water test kit (such as the AquaCare Aquarium/Pond Test Kit https://www.fishkeeper.co.uk/aquacare-master-test-kit )  rather than relying solely on visual cues. Clarity doesn’t always mean quality.

AquaCare Aquarium/Pond Test Kit at Maidenhead Aquatics

3. Leaves, debris and pond maintenance

What’s changing?

Autumn leaves can clog filters, block pumps, and decay in your pond, negatively impacting water quality.

What you should do:

  • Install a pond net or cover before heavy leaf fall begins.
  • Skim the surface regularly and remove any visible plant debris.
  • Check and clean filters, pump intakes, and UV clarifiers to maintain flow and efficiency.

Pro Tip: Cut back pond plants like lilies or reeds now to reduce organic waste and algae growth.

Leaves, debris and pond maintenance at Maidenhead Aquatics

4. Indoor aquarium adjustments

What’s changing:

As daylight shortens, indoor tanks may get less ambient light, affecting plant growth and fish behaviour.

What you should do:

  • Review your aquarium lighting schedule – you may need to increase artificial lighting to compensate.
  • Check your heater to ensure it maintains a steady temperature, especially in cooler rooms.
  • Begin planning any aquascaping updates or tank cleanouts before the busy festive season begins.

Indoor aquarium adjustments at Maidenhead Aquatics

5. System checks and equipment prep

What’s changing:

September is a great month to assess your outdoor setup before colder weather makes maintenance harder.

What you should do:

  • Check seals, pipes, and fittings on filters and pumps for wear.
  • Clean and backwash filter media (but avoid over-cleaning and removing beneficial bacteria).
  • Plan any major system upgrades or servicing now, while access and weather conditions are still favourable.

Expert Tip: Add a battery backup or air pump to aquariums to safeguard against potential power outages in stormier autumn weather.

System checks and equipment prep at Maidenhead Aquatics

Bonus Tip: Start planning for overwintering now

It’s not too early to prepare for winter. If your pond includes sensitive species like fancy goldfish or koi, you’ll need to consider:

  • De-icers or pond heaters
  • Moving sensitive fish indoors
  • Floating thermometers

Final Thoughts: September is your window of opportunity

September may feel like the calm before the storm, but it's one of the most important times of year to prepare your fishkeeping setup for the cooler, less predictable months ahead.

By adjusting your care routine now — before temperatures dip further — you'll ensure a healthy environment for your fish and reduce the risk of seasonal problems later.

Need supplies or advice for your pond or aquarium this autumn?


Visit us in-store or browse our expert-recommended seasonal care range online. From autumn food to water treatments, we've got everything you need to keep your fish thriving into autumn and beyond.