The best aquarium thermometer is the one that you can rely on. In general, aquarium thermometers are rather inexpensive pieces of equipment but help monitor one of the more critical aspects of your aquarium. Surprisingly this inexpensive device is often overlooked. Some reefers assume that their heater is calibrated accurately and if it’s set to heat to 80 degrees then that’s what it’s doing. Heaters are one of the most failure prone pieces of equipment in your tank. Assuming it’s accurate is rather silly really though I’ll admit I’ve done it myself. Also, everything in a reef tank needs a backup. Relying on your heater, even if it is accurately calibrated, as the only source of temperature monitoring is a recipe for disaster eventually.
External Sticker or Stick On (LCD) Thermometer
In-Tank Glass Thermometer
In-Tank Digital Thermometer
These thermometers are quite popular with fresh water aquariums for some reason. They aren’t bad although they aren’t the most accurate of options either. They sometimes can also be a bit difficult to read. Sticker thermometers work via a liquid crystal display that changes color with the temperature. Room temperature could affect the accuracy and placement should not be near the aquarium heater nor an HVAC vent, radiator, or direct sunlight. That said, you really shouldn’t have your aquarium near an HVAC vent, radiator, or in direct sunlight.
The inexpensive sticker thermometer isn’t the most accurate or easiest to read in general but this one rises above the rest in this category.
The tried and true traditional glass thermometer is quite an accurate and reliable. A solid choice with the caveat that it is made of glass care must be made to ensure that it won’t break. Make sure the method used to secure it is solid. Reef inhabitants could bump it or a rock could fall onto it.
If you’re going to go with a glass thermometer this is a great one to go with. It is a floating type so it will be best suited for a low flow area of your sump. The bonus with this one is that it measures salinity as well as temp. Pretty slick concept if you ask me. Also, it’s only a couple of dollars so if you have a good low flow location for it I couldn’t recommend it more.
Remain calm. Assuming the thermometer liquid was red instead of grey you shouldn’t need to worry much. Grey would be mercury and that would be an issue requiring you to drain your tank, remove your substrate, clean your rock, etc. etc. Chances are however that it was red in color which means it was simply alcohol. The small amount of alcohol isn’t going to do much so don’t sweat it. Do your best to remove all of the glass. If you need, scoop up the substrate in the area the thermometer broke to be as certain as possible that you got all the broken shards of glass out of there.
The gold-standard is the Digital Thermometer. Accurate, easy to read, and not easily breakable. Plus there are many options with the ability to set an alarm if it falls outside the desired temperature range. If you use the thermometer with an aquarium controller you can trigger events based on temperature fluctuations. That is really powerful. You can even have it send you a text message as an alert rather than a simple audible sound beeping that can only be heard when you’re home. If you decide on any digital thermometer I would highly recommend avoiding battery-operated ones. A low battery can give an inaccurate reading and swapping batteries is a minor headache that is easy to procrastinate doing.
There are many varieties of cheap digital thermometers available. Some as cheap as $3! I doubt a $3 digital thermometer will last a reasonable amount of time but for $3 what is a reasonable amount of time? 3 months?
If you want a good quality digital thermometer without going all out on a reef controller an inexpensive option that is very powerful is the Inkbird Pre-Wired 308S. This powerful tool allows you to monitor a maximum and minimum temperature and control 2 outlets based on these conditions. It’s basically a mini reef controller that only monitors and reacts to temperature. It gets knocked for not being terribly easy to use. Last, there are no batteries to replace.
In conclusion, what is the best aquarium thermometer? The best suggestion is to get more than one. Given how inexpensive they are, it’s a good idea. All things in your reef should have a backup. It also is helpful to verify the validity of your digital readings with an analog backup.
More specifically, having an analog thermometer paired with a digital thermometer that works with a reef controller is ideal. I won’t get into the vast benefits of an aquarium controller but suffice it to say that being able to make automatic adjustments based on temperature, such as turning on/off heaters or chillers, is incredibly powerful. You could even have it send you a text message when something goes wrong. It’s that just too cool? Your budget (spouse) will likely limit you from getting too wild. Good luck and happy reefing!
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