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Maintaining Aquarium Temperatures for Fish Health

Cichlid with ichMaintaining proper aquarium temperatures is essential for the health and well-being of your fish. While aquarium heaters do a pretty good job at this, the probability of fluctuations from fall through spring tends to be greater and possibly more detrimental. You may not even realize how much the temperature of the water changes through the day or day-to-day until you’re faced with ich or some other problem in your aquarium.

The Threat of Cool Temperatures

While our aquarium fish will rarely if ever be exposed to near or below freezing temperatures in the safety of your home (hopefully), fish farmers in Florida can attest to the immediate and lingering problems that can come with even short exposure to cold temps. Exposure to temps below 60 F can create chaos in a tropical tank, so you can imagine what freezing temps do to tropical fish housed in an outdoor setting. Sensitive fish may be killed outright from the shock of extreme temperatures or fluctuations in temperatures. Others face blows to their immune systems and the increased chance of being infected by opportunistic parasites, fungi or bacteria. These organisms take advantage at the slightest sign of stress on the part of tropical fish, and can decimate the population in a short amount of time. Cooler temperatures tend to make normally active fish lethargic and slower to react, making them more open to predation if outdoors. Similar problems can occur in the aquarium if smaller or more sensitive fish are not able to hide or escape the curiosity of larger, hardier tankmates. 

What you can do

First thing, know what the ideal temperature range is for the fish you keep. For example, if you keep fancy goldfish, you’ll want to maintain the tank at between 60 and 75 F. But goldfish can generally tolerate much cooler temps and daily fluctuations with ease. Neon tetras would prefer temps maintained at between 68 and 80 F, and may be much more sensitive to even slightly cooler temps and small fluctuations. Monitor your tank’s temps with a reliable thermometer and try to observe the changes in temperature through the day (and after the lights go out).

Neon TetraThe standard means of heat regulation in the aquarium is with the use of aquarium heaters. 3-5 Watts per gallon is generally the recommendation for supplying adequate heat. However these heaters only increase the temperature of the aquarium a certain number of degrees. If the aquarium’s room is drafty or kept cooler than the rest of the house, 3-5 watts per gallon may not be sufficient or efficient to keep the temperature in the desired range. Performance of the heater depends on many aspects including wattage, the size of the tank you put it in (larger tanks may require multiple heaters to maintain even and consistant temps), and the temperature of the room where you keep the tank. Beware of vents, windows and doorways in the tank room. Frequent blasts of warm and cold air and consistant drafts can quickly impact an aquarium, especially smaller set-ups.

Keep it Cool?

Angelfish and fryRemember that too much heat can also be your enemy. Particularly reef aquariums and those containing invertebrates must be monitored carefully (especially during very hot summer months) to ensure that the temps don’t rise to rapidly or stay too high for extended periods. Chillers are often employed by serious reefers to protect against high temps, but there are some other ways to keep a tank cooler if you can’t afford one. But even for freshwater and tropical species, higher than average temps of maintaining constant temps at the higher end of the ideal spectrum can have less desireable effects.

Many breeders believe that keeping certain species of fish in the lower range of their temperature tolerance is advantageous.  Cooler temps can stop prolific pairs from spawning, preventing unwanted fry. It is also believed that the lifespan of your fish can be increased when kept in the low end of the temperature tolerance range, possibly even 2-4 times the normal lifespan. Growth rate also slows, but the longer the fish lives, the longer it will grow ultimately resulting in larger max size. Some aquarists swear they see reduced aggression in territorial species kept in cooler tanks. These tricks are not for the novice. Care has to be taken in maintaining this lower range; be sure to observe your fish carefully for any signs of distress. You may also need to adjust feeding amounts and frequency as their metabolic rates will likely drop and it can be very easy to overfeed.

Natural fluctuations in aquarium temperature through the day are normal. The idea is to keep the changes slow and within a reasonable range, thus reducing the stress on your fish population as a whole. By being diligent in keeping the temperature withing desired range, you help to keep your fish happy and healthy.



Angelfish and Fry image referenced from wikipedia and originally posted by Hodowlaniec


  1. avatar
    Marine Reef Aquarium

    Thanks so much for this post. You’ve shared so much insight and information that is greatly appreciated.

  2. avatar

    I don’t heat most of my aquariums, but I like to keep heaters kicking around in case I need to control the temperature for breeding or fry. 🙂

  3. avatar

    Really great post on aquarium temperature! I always keep a heater in my tank to maintain proper temperature. Plus, my house can get kind of cold here in the Winter!

  4. avatar

    Hello! I’ve been following your site for a long time now and finally got the bravery to go ahead and give you a shout out from Humble Texas! Just wanted to say keep up the great work!

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