What "cold water" really means
A cold water aquarium is really a temperate aquarium: it is kept for species that are comfortable at normal indoor room temperature, roughly 18–22°C, rather than the warmer conditions tropical fish need. The defining feature is that it runs without a heater, relying on your home's ambient temperature instead.
That is genuinely one less piece of equipment to buy and manage, but it does not make the tank low-effort overall. Everything else — filtration, cycling, water changes and sensible stocking — works exactly as it does in a tropical tank. In fact, the most popular cold water fish, goldfish, are among the messiest in the hobby, so filtration and volume matter more, not less.
Equipment you need (and don't)
A cold water setup is a standard aquarium minus the heater. Prioritise volume and filtration.
- A tank as large as you can sensibly fit — see the aquariums hub. Goldfish in particular need serious volume.
- A filter rated above your tank size, because temperate messy fish produce a heavy bioload. Compare types on the filters hub and in our filter guide.
- A thermometer to monitor stability.
- A liquid test kit, dechlorinator, and a substrate and decor of your choice.
- No heater — unless your room drops very low, in which case a modest one just prevents big dips.
Cycle the tank first
Cold water tanks must be cycled before adding fish, exactly like any other. Beneficial bacteria still need to establish before livestock is exposed to ammonia and nitrite. One quirk: bacteria multiply a little more slowly at cooler temperatures, so a temperate fishless cycle can take slightly longer than a warm tropical one.
Follow our full cycling guide — dose an ammonia source, test daily with your kit, and wait until ammonia and nitrite both read zero within 24 hours. Patience here prevents most early disasters.
Choose hardy, temperate species
Stock only fish genuinely suited to cool, unheated water, and research the adult size of everything before you buy.
- White Cloud Mountain minnows — peaceful, hardy, cold-tolerant schooling fish ideal for smaller temperate tanks; keep them in a group of six or more.
- Goldfish — characterful but large, long-lived and messy; they need a big, well-filtered tank, and common single-tailed types really belong in ponds.
- A Marimo moss ball and other cold-tolerant plants add greenery and help absorb nitrate.
Stock lightly and settle in
Once cycled, add fish gradually — a few at a time — so your filter bacteria can grow to match the load, and acclimatise each new arrival slowly using the method in our acclimation guide. Understock rather than overstock; a lightly loaded temperate tank stays far more stable.
From there, maintenance is the usual rhythm: weekly partial water changes, occasional testing, and sensible feeding. Because goldfish and other temperate fish are heavy waste producers, keep an eye on nitrate and lean towards larger or more frequent changes if it climbs. The maintenance hub has the full routine.