Most marine systems are set up to house tropical livestock, which means that attention has to be given to maintaining a moderately high seawater temperature. The recommended temperature is 75 to 80 deg F, though some experienced aquarists run at a higher level to increase the metabolism of all the livestock. The novice should not do this but stick to the recommended levels.
Heating the seawater is straightforward nowadays, as a unit called a heater/stat is commonly available. These take the form of a longish tube with a thermostat at the top end and a heater unit at the lower end. They are available from local retailers and online and the price is very reasonable.
Aquarium heaters used to be the source of a few problems in the past with sticking contacts in the thermostats or general failure. Nowadays they are more reliable though the danger of a sticking thermostat is still present. There are ways of reducing the risk but obtaining the heater unit is the first consideration.
The heater/stat is available in several wattages (W) and the size of the aquarium, or rather the net total gallonage of the aquarium which includes any sump, dictates the heating need. The usual guideline is to allow 2 watts per gallon of seawater for a normally heated room. If the room is unheated then the wattage is doubled. So keeping things simple, a 50 gallon system in a heated room would require 100W, and in an unheated room 200W. These wattages are not based on a formula that must be rigidly followed, what is required is that the heating is known to be fully capable for all possible demands. Wattages that are a little higher – within reason – should not be harmful.
It may be thought that buying a bigger heater, say 400w in a heated room, would be better as that capacity will not face any problems ever. However, it is not a good idea. If the thermostat contact sticks in the ‘on’ position then the seawater will heat up more quickly, possibly meaning that the aquarist doesn’t notice the excessive temperature problem until it is too late.
Once the heating requirement is known there isn’t anything to stop the aquarist purchasing a heater/stat of the correct wattage (if the wattage doesn’t match the net gallonage the next heater up should be chosen). Again remembering that though modern heaters are much more reliable than they used to be potential problems have not been totally eliminated, so it is best to obtain two heater/stats. They should not individually equal the full heating requirement of the system, but half of it. In other words, if 200W is required then obtain two heater/stats at 100W each. This will help to prevent a problem with one of the thermostat contacts sticking ‘on’ or ‘off’: if one sticks in the ‘on’ position then the other will turn off when the design temperature has been reached slowing down the temperature climb, or if it sticks in the ‘off’ position then the other heater will prevent the seawater cooling too quickly. Hopefully the aquarist will notice the problem in the extra time the system affords.
With two heater/stats the temperature setting on each should be the same. Many heater/stats are set at 75 deg F before they leave the factory so if the aquarist requires a higher temperature then the devices will require adjustment. This brings up another point: when purchase is being considered ensure any temperature adjustment is easily accomplished. There are heater/stats nowadays that display the temperature setting on a clear scale and an easy to use adjustment knob is at the top of the unit.
There isn’t an absolute requirement that two heating units are used, but for the reasonable extra financial outlay it is worthwhile.
Finally, before parting with hard cash find out what the temperature variance of the heater/stat is. The temperature variance is the difference between when the heater turns off and when it turns on again – in other words the sensitivity of the thermostat. A variance of 1.5 deg F is good but manufacturer’s products vary.
Really that is all there is to consider when obtaining heater/stats, but I’ll just mention one other thing. I should point out that though this is generally desirable for accuracy and dependability there isn’t an absolute requirement.
Some aquarists when setting up a system decide to employ an exterior electronic controller. They can of course be retro-fitted. The controller usually takes the form of a small box perhaps 4″ square or so. There is a mains electricity connection, and a socket from which the electricity for the heaters is supplied. In addition, there is a small temperature probe that goes in the seawater. Temperature is set usually by means of a small knob, and the heaters are shown to be active or not by an indicator light. If two heater/stats are in use, they should both be connected to the controller (ensure the power handling capacity – wattage – that the controller can handle is not exceeded). Once the design temperature for the seawater has been set on the controller, the temperature setting on the heater/stats should be 2 deg F higher. This means that the heater stats are always in the ‘on’ position. Some electronic controllers pulse electricity through the heaters, reacting to the probe readings. If the seawater is trying to cool, the pulses are longer (so the heaters apply heat for longer), if it starting to warm up too much the pulses shorten. It is reported that as the heater coils are always warm, not going from cold to hot and back again the heating coils last longer. These electronic units could control temperature to +/- .5 deg F. It sounds as though they could be an expensive item, but they aren’t.