First thing, know what the ideal temperature range is for the marine 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).
The 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.
Remember 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 saltwater 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 marine 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.
