Finnex HMH-300S 300 W Digital Titanium Aquarium Heater review

A longer tube for Finnex’s 80–100 gallon row

The HMH-300S is Finnex’s 300 W internal-sensor titanium configuration, listed for 80–100 US gallons. Its 12.80-inch tube is the second longest HMH size in the manual, after the HMH-500S at 15.55 inches. This model begins where the 200 W row ends: both share the 80-gallon boundary, but the HMH-300S carries the 80–100 gallon range and its own power/cable values. A tank-volume label alone does not establish the heater’s physical fit, so measure submerged room and check the exact suction-cup orientation before choosing the model.

The source table specifies 2.70 A at 115 V, a 55-inch power cord, a 39-inch controller-to-heater cord and a 47-inch sensor cord. The shorter control lead compared with the 500 W row may affect where the external display can sit, but it should not be extended or rerouted against an unverified electrical instruction. The manual table does not state a complete voltage/frequency nameplate, and the inspected current official Shop pages did not resolve a model-specific HMH-300S retail SKU, UPC or variant. Check the exact package rather than treating the family table as proof of an available offer.

Internal sensing and a limited dry-protection window

The HMH-300S reads through its built-in tube sensor. Finnex identifies the junction at the heater cord as the sensor location and says it should remain higher than the rest of the element. HMS units instead use an external probe; their separate setup instructions should not be applied to this family. For HMH, the manual recommends a spot near moving water, secured to glass with suction cups, with no pressure on the tube ends or cord. It also permits placement in a filter sump, provided the heater remains fully submerged.

Its digital control spans 50–94°F in tenth-degree increments. Finnex states the controller display is accurate to ±0.5°F, but that does not certify the aquarium’s water as stable at a particular temperature. Use an independent thermometer. There is a consequential interaction between the broad setting range and low-water safety: the manual says the low-water warning and cutoff function from 58–92°F only. Outside that range, the system is disabled and the heater can continue heating when its sensor is out of water. A high-temperature alert above 95°F is a warning to unplug and investigate, not proof that the unit has independently maintained a safe tank.

Plan installation before switching it on

Finnex’s instructions call for completely immersing the HMH-300S, allowing 15 minutes for the heater to approach water temperature before power, using a drip loop, and connecting through a GFCI-protected receptacle. Unplug aquarium equipment before maintenance and allow the heater to cool for at least 15 minutes before removal. Check and clean the sensor every 30 days as directed. These are not discretionary accessories: the sensor’s location, circulation and power routine are part of the manufacturer’s stated operation.

Read the 80–100 gallon band as a boundary

The 100-gallon top is a maximum, not an all-purpose heating guarantee. Room temperature, tank cover and water circulation can change the heat load; Finnex’s table does not convert the range into a promise for every environment. The 80-gallon lower endpoint also matters, but the current finder only applies maximum-capacity rules. HMH-300S stays reference-only rather than being shown as a suitable 1–100 gallon option. Finnex says the family is for freshwater and marine aquariums where TDS is above 150 ppm. The control range, exact electrical package, current sale configuration and photo identity should be verified separately.

For exact identification, compare the manual label HMH-300S, its 300 W rating, the 80–100 gallon band, 12.80-inch tube and 39-inch controller lead. This is a source-based model assessment, not an independent test or proof the heater is right for a particular aquarium’s livestock or target temperature.