Spacesaving design for small tanks: ultracompact and does not occupy space outside the tank. Ideal for mini and nano aquariums with a water capacity up to 4 L. Universal compatibility: suitable for both freshwater and seawater systems. Nickel heating wire with sandfilled construction: a type of nickel soldering electric heating wire surrounded by sand to efficiently transfer heat, fully converting heat energy, quickly accumulating energy, and helping save electricity. Compact and portable: as small as a thumb, easy to hide and position in micro tanks without crowding the setup. Precise temperature control: USB microtype cylinder heating rod delivers accurate temperature regulation from 18 to 34 C, with 0.1 C precision to maintain the desired environment for fish. Use scenarios Desktop nano tank setup: place this heater in a 2–4 L freshwater or seawater nano tank to maintain a stable 24–26 C, supporting tropical fish or small invertebrates without adding bulk. Small marine or freshwater quarium: use in a 4 L max tank to keep ambient temperature steady for sensitive species, reducing temperature fluctuations and stress. Fry or breeding tank: keep a dedicated microtank at a precise 26–30 C range for small fry or breeding setups, ensuring a controlled environment while preserving space for observation and care.