A nice warm summer is that time of year most people dream about here in the northern hemisphere, but for us aquarists it can be a curse.

Temperatures skyrocket, sometimes well into the 90’s and this can make life very difficult for us marine aquarists, as many of the animals we keep are very sensitive to high temperatures and prolonged exposure to it. To remedy this some people use a chiller while others rely on something more economical such as fans of various kinds to bring down water temperature. Leaving the cover glass off is also a very effective method of bringing down temperatures, especially when combined with a fan blowing over the water surface, but that’s not without its own problems. Let me give you a quick overview of the pro’s and con’s of the different methods here.

Open top
Pros: Does not trap the heat and allows it to freely disperse, as well as improving gas exchange which in turn lowers CO2 levels in the water and improves oxygenation. In addition you get more out of your aquarium lights than if you were using a cover glass which would diffuse the light.
Cons: Greatly increased risk of certain species of fish going carpet surfing! I.E Jumping out. Causes a significant increase in evaporation which may promote significant swings in salinity, especially in smaller tanks.
Comments: My honest opinion is that the Pro’s greatly outweighs the cons. If you are afraid of fish jumping out it is possible to build or buy a mesh screen for the top of your tank will very effectively keep your fish from jumping out. The concern of evaporation is also easily remedied with an Automatic Top-off System(ATO) with RO/DI water which will maintain a constant water level and stable salinity. An open top can also benefit considerably from a fan moving air around.

Pros: Very cheap, especially if its a DIY build. Increased airflow around the tank and over the water surface helps heat dissipate.
Cons: Increases evaporation significantly.
Comments: Some fans can be mounted on the tank, but a floor fan in the room or aimed at the tank can also work very well, especially if you don’t have a canopy or cover glass on the tank. Because of the massive increase in evaporation, an ATO is highly recommended.

Pros: Very effective and reliable way of bringing down aquarium temperatures even when dealing with extreme heat. Works well with closed top aquariums. Does NOT increase evaporation.
Cons: Very expensive, requires a lot of power.
Comments: I don’t have any personal experience with them, but what I have learned is pretty much this: Don’t settle for the cheapest, as they are often inadequate and the more powerful ones tend to give you a better bang for your buck. Do your research properly and determine exactly what model meets the needs for your aquarium.

Also keep in mind that ALL electrical equipment for your aquarium generates heat. Metal Halide lighting being the most notorious offender.
Now this is by no means a comprehensive guide, and you would do well to do further research on your own, but hopefully this little blog post will have pointed you in the right direction.

In an emergency like mine where you have a cover glass that needs to come off but don’t want your fish jumping out, its always good to improvise. Some mosquito netting and a roll of tape saved the day here.