A common perception is that goldfish are coldwater fish and therefore do not require a heater. Whilst this is true of the common single tail variety, fancy goldfish do better if the tank is maintained at around 22deg C (72deg F).

The purpose of a heater in a goldfish tank isn’t so much as to heat the water up but to ensure it stays above 22c and doesn’t wildly fluctuate in temperature. This is especially important over winter nights when air temperatures can reach bitter cold before warming up again during the day.

Heater Strength

When choosing a heater strength take into consideration how cold it will get where the tank is being housed. It might be 10 degrees outside in the middle of winter but if you’ve got heating in the house keeping the air at 20deg, then you don’t go overboard in heating your tank.

If you intend to keep a large tank often it’s advisable to keep two smaller heaters then one larger one in case of failure. My house is relatively well insulted and I find there isn’t much of a temperature swing so I only use one heater. Should I ever move to a not so well insulated house though I’ll definitely be purchasing a backup heater in case of failure.

The more your air temperature swings during the day and night the more critical heating your goldfish tank becomes to maintain a steady temperature.

Where to place an aquarium heater

Generally speaking the wattage you choose will dictate heater placement, as watts increase with the units so does length. Universally heaters tend to be mounted on the back walls of tanks and can be hidden behind plants, driftwood or ornaments.

The best placement for a heater I’ve found is either near the intake of your filter or near the output nozzle. The reason for this being that it ensures the best water movement of the heated water and results in greater temperature consistency around the tank.

A Fluval Tronic heater cover

A Fluval Tronic heater cover

It is important when placing the heater to ensure that fish can’t get trapped between the heater and the glass. Goldfish can be persistently stubborn at times and getting lodged between a heater core and the tank wall isn’t unheard of (or pleasant to clean up afterwards).

If you’re still concerned about your goldfish potentially getting stuck behind a heater you can also buy heater guards for most models. I know that Fluval’s Triptronic range have a set of corresponding heater covers (sadly at an additional price) and I’m sure most other manufactures have options available.

Running Cost

A lot of people assume that an aquarium heater is always on and therefore consumes a huge amount of power of time. This is not true.

Like an air heater, a fish tank heater is not always on, it stays off until the water drops below a preset temperature on the unit. Once this temperature threshold is triggered the heater turns on and warms the temperature to this set unit and then shuts off again.

Aquarium heaters vary in power from 25w to 300w+. Three things to consider about the cost of running an aquarium heater are:

1. Most household electric heaters are in the vicinity of 2000w and are required to heat up and regulate much larger air spaces then an aquarium. An aquarium heater is usually around 1/10th the capacity of this and is in a much smaller environment.

2. Water holds heat twenty four times better then air, so heating efficiency is greatly increased in an aquarium environment.

Monitoring your temperature

An in-tank thermometer makes for a great inexpensive fail check to ensure your heater isn’t broken. I have one discreetly tucked away behind my output hose on the side of the tank so I can glance at it as I walk past.

With glass sealed thermometer units costing only a couple of dollars I don’t see the point in going the digital LCD route which will need batteries replaced every so often but the choice is yours, either will do a good job reporting the temp.

Heater help when treating goldfish for sickness

A temperature of around 25 deg C (78deg F) is ideal when treating goldfish of many sicknesses. At this temperature their immune systems are at their peak and the life cycle of a lot of nasties are sped up ensuring faster kill time when medication is being used.


Heaters certainly aren’t required for a goldfish tank but do make a difference to their comfort levels in regulating water swings. Whilst it is true that single tail goldfish are a coldwater species their fancier cousins do not occur naturally in nature, are generally weaker and benefit from a warmer temperature.

Personally I use a Fluval Tronic 300W on my 255L Walstad tank and have had no dramas whatsoever.  You do pay a premium for the Fluval heaters but similarly to heating a house I believe money spent on a good heater is money well spent.

There are cheaper options on ebay but they can be prone to dodgy thermostats that lock in the ON position and will heat up the water until you notice it’s stuck on. Obviously this isn’t good for the goldfish. In addition to this because they are completely submersible cheaper parts can also leak current into the water which is obviously not going to benefit your tanks inhabitants.

Heaters aren’t serviceable so personally I think it’s better to buy a branded one and resist the ebay $10 jobs from Hong Kong.