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What aquarium volume does Finnex list for the HMS-300S?
The exact HMS manual lists a 80–100 US-gallon range for HMS-300S at 300 W. That is a manufacturer band, not an independently tested guarantee; the max-only finder cannot enforce its 80-gallon minimum. The product therefore remains outside finder matches rather than being treated as suitable for every smaller volume.
Explore this need in the finder →How does the HMS-300S control temperature and sense water conditions?
The HMS manual documents a 50–94°F adjustable digital setting with 0.1°F increments and stated display accuracy of ±0.5°F. That is the maker’s display specification, not a measured stability result. The HMS-300S uses an external water sensor separate from the titanium tube. Finnex’s HMS manual says to position the probe slightly above the tube, just below its clip without slack, and keep it submerged near the heater; this is different from HMH’s built-in sensor arrangement. The family page also requires water above 150 ppm TDS.
Explore this need in the finder →Can the HMS-300S be installed inside an aquarium?
The HMS manual requires the heater itself fully submerged in the aquarium or sump, secured with suction cups in moving water, and its remote sensor underwater near and slightly above the tube. Allow 15 minutes for temperature acclimation before power; use a GFCI outlet and drip loop. Within the manual’s 58–92°F low-water protection range, exposure stops heating and sounds an alert; outside that range the low-water system is disabled and the manual warns the heater may continue heating. The manufacturer describes freshwater and marine use but requires water above 150 ppm TDS; verify water chemistry, exact supply rating and exact package instructions.
Explore this need in the finder →A 300 W row with a 12.40-inch tube
The HMS-300S is Finnex’s 300 W remote-sensor heater for a stated 80–100 US-gallon aquarium. Its titanium tube measures 12.40 inches, shorter than the 500 W HMS-500S’s 15.16 inches and a little longer than the 200 W HMS-200S’s 11.22. The 80-gallon lower edge overlaps the 200 W band’s upper edge; the exact wattage, capacity interval and tube length distinguish the two. Check physical immersion and sensor placement in the specific tank instead of assuming that two rows touching at 80 gallons are interchangeable.
The manual table lists 2.70 A at a 115 V reference, a 55-inch power cord, 39-inch controller-to-heater cord and 47-inch sensor cord. The controller lead is longer than the 200 W model’s 35.4 inches and shorter than the 500 W model’s 47 inches. These values can aid setup planning, but the heater must not be installed where the cords are stretched or water can reach the outlet. Full voltage and frequency are not established by the table. The exact current retail SKU, UPC and Shop configuration were not verified for HMS-300S.
Keep the remote probe in the right location
HMS measures with a separate remote water sensor; HMH puts its sensor in the tube. Finnex instructs the HMS user to position this probe underwater slightly above the heater, directly beneath its cord clip with no slack. The separate probe is part of the safety setup, not an optional temperature accessory. It must remain near the tube and submerged while the tube is secured to glass with suction cups, fully immersed and placed in free-flowing water. The manual suggests proximity to a filter or wavemaker and permits use in a filter sump. It does not establish inline external-filter compatibility.
The digital controller can be set from 50–94°F in tenths of a degree. Finnex states ±0.5°F display accuracy; that figure is not a measured claim for water in a particular tank. Confirm actual temperature with a separate thermometer. The low-water cutoff/alarm has a narrower operation range: the manual says it functions from 58–92°F and is disabled outside those settings, with a warning that the heater can continue heating when the sensor is exposed. If the controller’s above-95°F audio alert sounds, the manual says to unplug and investigate. These statements should be read together rather than assuming every setting has the same protective behavior.
Consider the whole installation routine
Let the unit acclimate in aquarium water for 15 minutes before plugging it in. Use a GFCI-protected receptacle and arrange a drip loop. Unplug aquarium equipment before cleaning or moving the heater, then wait 15 minutes for cooling. Finnex calls for cleaning and checking the remote sensor every 30 days. If the low-water system activates, the remote probe must be submerged again and the controller reset as directed. An independent thermometer and routine water-level check remain necessary; the display, probe and cutoff do not verify a biological target or a specific tank’s heat load.
Conservative status reflects the missing minimum filter
The HSM-300S row begins at 80 gallons and ends at 100. The current questionnaire’s numeric rule filters only its maximum, and no evidence from a 100-gallon maximum should be used to imply the lower limit is optional. This configuration remains reference-only, outside active matches. Finnex says HMS is usable in freshwater or marine systems where TDS exceeds 150 ppm. The exact full electrical nameplate, current individual offer and wattage-specific photograph were not resolved. The model’s manual facts remain useful for checking its distinct tube and probe arrangement, but do not amount to testing or a recommendation for a particular aquarium.
Key specifications
Key details at a glance
| Rated heater power | 300 W |
|---|---|
| Lowest thermostat setting | 50°F |
| Highest thermostat setting | 94°F |
| Temperature control | Adjustable thermostat |
| Supported water types | freshwater / saltwater |
| Installation route | Fully submerged, in-tank |
| Heater length | 12.4 in |
Use the exact model’s instructions for operating limits, supplied equipment and version differences.
Paper, software and setup details
Manual-recommended aquarium range
80–100 US gal
Sources for this review
Sources for this assessment
Check the complete aquarium setup
Use the finder to compare documented tank size, water type, installation and temperature-control fit. A match does not guarantee the keeper's species-specific target or measured performance.