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What aquarium volume does Finnex publish for THD-300S?
Finnex's THD-300S table gives a 40–80 US-gallon band. The lower 40-gallon endpoint is not just a note that can be ignored: the unchanged numeric finder can enforce the 80-gallon ceiling but cannot enforce that 40-gallon floor, so this model remains reference-only rather than appearing for smaller aquariums. The published range is not a guarantee of a particular temperature rise.
Explore this need in the finder →Does a standalone THD-300S include temperature control?
No controller is shown as included in Finnex's THD Shop selector; the page describes the item as a heating element only and states controllers are sold separately. The THD table provides no built-in setpoint range. Verify that the exact offer includes a controller you can use, its load rating and the complete heater/controller instructions before assembling a system.
Explore this need in the finder →How does the THD-300S differ from Finnex's TH-300S?
Both exact 300 W rows have the same 40–80 US-gallon band, but Finnex lists THD-300S at 0.62-inch diameter and 12.5 inches long, while TH-300S is 0.75 inch and 10 inches. Finnex says the TH design has extended quartz sleeves and coils compared with THD. Those are manufacturer design descriptions, not a measured durability or heat-distribution result, and both heater elements need separate controller verification.
Explore this need in the finder →What the THD-300S specification actually identifies
This is an evidence-based assessment of the Finnex THD-300S, not a report of personal use, testing or independently measured temperature. Finnex's TH/THD comparison table gives the THD-300S a 300 W rating, 120 V, 2.7 A, a 0.62-inch diameter, a 12.5-inch length and a 40–80 US-gallon band. The separate current Shop record offers a 300 Watt option with Shopify variant ID 51093135687979 and the SKU written “THD-300s”; the manufacturer family table writes the model suffix with an uppercase S. That capitalization difference is preserved rather than hidden. The observed UPC/barcode field is empty, and no separate retail UPC is asserted.
The size range is two-sided. The manufacturer's 40-gallon lower limit matters as much as the 80-gallon ceiling. Our existing numeric finder can compare a tank's volume with the maximum, but it cannot require that a tank be at least 40 gallons. A lower-volume aquarium could otherwise pass a max-only check even though it is outside Finnex's displayed band. That is why this review remains in the library as a reference but the product does not appear as a matched buying recommendation.
THD-300S is not the same tube as TH-300S
Two Finnex models at 300 W share a 40–80-gallon listing, but they are not duplicate names for one configuration. Finnex gives the THD-300S a slimmer 0.62-inch body and a 12.5-inch length; the TH-300S has a 0.75-inch diameter and a shorter 10-inch length. The manufacturer describes the THD as a titanium element with an internal quartz sleeve. Its page contrasts the TH line's longer quartz sleeves and coils with THD's more compact construction. Those are documented design descriptions, not a test result showing which heater lasts longer or distributes heat better in an aquarium.
Physical fit and capacity are separate questions. Twelve and a half inches describes the listed tube length, not a clearance measurement or an installation direction. The available source table does not establish how much space is needed at the end, where a cord or sensor should sit, whether a specific orientation is permitted, or an immersion line. A tank that can physically contain a tube is not automatically an approved setup. The exact supplied instructions should resolve placement before the element is used.
A heating element is not the whole temperature-control system
Finnex's THD Shop description calls the item a heating element only and says controllers are sold separately. The model's own specification table does not state a thermostat range, display, probe, digital controller, power-loss memory or dry-operation feature. None should be borrowed from Finnex's HMU, STE, HMC or other controller-equipped products. The basic purchasing task here is therefore more involved than selecting a wattage: a compatible external control unit is another component, and the reviewed pages do not establish which particular controller is bundled with this selector because the selector is for the element.
Before comparing a real offer, verify that it names THD-300S, includes or clearly identifies the separate controller you intend to use, and supplies complete directions for that exact pairing. Check the electrical load against the controller's own documentation and inspect the heater and controller labels. The family page reports 120 V and 2.7 A in separate columns, but it does not explicitly describe the amperage reference condition; frequency is not stated. I retain the two source values as listed and do not calculate an alternative current rating or infer a complete supply specification.
Material, guard and water-type claims—with limits
Finnex describes THD as a titanium tube with an internal quartz sleeve and says each TH and THD heater comes with a high-temperature protective guard. The same family page's titanium copy describes marine and freshwater use. These support a material description, a guard inclusion and the manufacturer's stated water types. They do not establish that the complete body is shatterproof: the page expressly mentions an internal quartz sleeve, and its general titanium language is not a detailed impact test of this exact assembly.
The manufacturer instruction index checked for this review lists several heater manuals but not a THD manual. That is a limited search result, not proof that no THD instructions exist elsewhere. As a result, this assessment does not state that the element may be operated dry, fully submerged to any particular mark, attached inline, installed vertically or placed in a specific flow zone. A guard is not a substitute for model-specific operating instructions.
Why the available photo and match status stay conservative
The official THD image used in the Shop selector shows a guarded tube and “COMPACT,” but no THD-300S marking or wattage; it remains a family-level illustration. A retailer listing photograph now supplies the exact model cue: its THD carton visibly labels “THD-300S” and “300W,” with the 300 W option marked, alongside the guarded element. That package photo is identified as a retailer image and is not a scale reference or performance evidence; the public record does not reuse it as proof of dimensions.
The practical record for this model is clear but incomplete: Finnex names a 300 W THD element, publishes its 40–80-gallon band and dimensions, and describes the titanium/quartz element and common guard. It also makes clear that the controller is separate. A maximum-only finder cannot honor the 40-gallon floor, and full electrical details and installation steps are still unverified. Those are concrete reasons for reference-only treatment, not a claim that the product is unsafe or unavailable. The retailer package photo now provides a model-specific 300 W identity cue, but it does not solve those fit or operating-information gaps. If considering a THD-300S, confirm the model and supplied parts, the actual controller, outlet requirements, tank-volume guidance and the exact instructions before relying on the assembly. The stated wattage and gallon band are manufacturer specifications, not a guarantee of a temperature result in a particular room or aquarium.
Key specifications
Key details at a glance
| Rated heater power | 300 W |
|---|---|
| Supported water types | freshwater / saltwater |
| Electrical supply | 120 V |
Use the exact model’s instructions for operating limits, supplied equipment and version differences.
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.