Product: Therma-Flow Heater Series
Manufacturer: Penn Plax
Price: $7-(25 watts) <--> $12-(500 watts)
Pros: Heater is inexpensive and does a very good job. Is easy to set, holds its temperature well and is very durable.
Cons: If you have no need for a submersible heater, then there are no bad points to this heater.


I may be wrong, but I believe that the Therma-flow series of heaters is the longest running series of heaters available for home aquariums. I owned Therma-flows on some of my earliest tanks 15+ years ago. This heater has become well known for its durability and accuracy among hobbyists. There are several sizes, from 6"-15" and the range in watts from 25-200. This heater can be used on tanks from 5-40 gallons. The retail price is $7-$12 with the larger more powerful heaters costing more.

Accuracy (9)

I’ve used the Therma-flow heaters for a long time and thus have lots of experience with them. These heaters are very good at retaining the temperature you set them to. Unfortunately you cannot set them to any specific temperature, however, the low price more than makes up for that fact. Many low-cost hang-on heaters have a propensity to shift temperatures slowly over time. For the most part, once you have gone through the initial adjustment period and set the heater to a temperature, you won’t have to worry about it again!

Ease of Use (8)

One of my least favorite points of hang-on heaters is the difficulty in setting the heater to the temperature you would like to use. Pre-set heaters that can be set to a specific temperature are obviously much easier! My second least favorite thing about hang-on heaters is that the knobs used to set the temperature are usually VERY touchy. On all my other hang-on heaters other than the Penn-Plax Therma-Flow heaters, any touch to the knob will turn the heater light on. That is very difficult to deal with as to set a heater that cannot be set to a specific temperature, you rely on the light to set the heater. However, on the Therma-flow, even my older flows-flows, the light always goes on when it is supposed to, and goes off when it is supposed to. This shouldn’t be a feature, but since it is so rare on hang-on heaters, it makes the Therma-Flow stand out as being much easier than other to use.

(If you’ve never done this before, here is the basics, you put the heater in, determine the water temperature and then turn the knob until the light just goes on. This will keep the tank at about the same temperature as it is now. You can then turn it slightly higher or lower to adjust the temperature. If you get lost, go back to where it is off, and then move until the light comes on again…all very simple as long as the light doesn’t turn on every time you touch the heater)

Reliability (9)

Perhaps the most amazing feature of this heater is its durability and reliability. Over time the metallic plates inside the heater that allow it to function properly fuse together. It always happens, it’s just a matter of time. When that happens, the heater will usually excel to high temperatures and cause mass destruction in your tanks. Though I’ve owned Therma-Flow heaters for several years, I’ve never had them overheat a tank. I’ve had one stop working all together, but never to my detriment. The heaters last a long time, usually 5+ years. They are also very water resistant. I’ve placed them in water that went over 1 inch above the water line on the heater, and still no water entered the tube. Due to their low price, you could replace them every other year if needed, but you certainly won’t need.

Overall (9)

As far as hang-on heaters go, there is frankly no competition. Therma-flow heaters are a very inexpensive option that will serve you very well. Though they do not offer the convenience of a pre-set heater, the price and long-term durability more than makes up for that lack. Unless you have an absolute need for a submersible heater, I would recommend this heater for all your aquarium heating needs.

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