How the aF4 Keeps Food Fresh — RSW TechnologyUpdated a month ago

The aF4 uses a food preservation method adapted from the commercial fishing industry known as RSW technology — Refrigerated Seawater Technology. Understanding how it works helps explain why high salinity saltwater is essential for dilution, and why the aF4 is able to keep frozen fish food fresh for extended periods.

What is RSW technology?

RSW (Refrigerated Seawater) is a method used aboard commercial fishing vessels to preserve freshly caught fish at sea. By chilling seawater down to approximately 0°C — just at or below freezing — fish can be kept in a cold, saline slurry that preserves freshness far longer than ice alone.

inD Aquatics has adapted this same principle for the aF4, scaling it down from large ship-board tanks to the aF4's 200 mL reservoir. The result is a system that can reliably store prepared frozen fish food in a cold saltwater slurry and dispense it on a daily schedule while maintaining freshness for days or weeks at a time.

Why saltwater — not freshwater

The saltwater component is not just about matching the salinity of your aquarium. It plays a direct role in how well the food is preserved.

Lower freezing point
Saltwater freezes at a lower temperature than freshwater. High salinity saltwater (1.026–1.030 specific gravity) freezes at approximately −2°C, compared to 0°C for freshwater. This means the aF4 can chill the slurry to near-freezing temperatures without the food becoming a solid block — keeping it in a pourable slurry state while remaining as cold as possible.

Preservation properties
The elevated osmotic pressure of high salinity water slows bacterial growth in the food slurry, extending freshness duration. Freshwater dilution lacks this property and will shorten how long food stays fresh.

Snowflake setting compatibility
On the snowflake (coldest) temperature setting, the unit chills the reservoir to −1°C to 0°C. Using freshwater or low salinity water at this setting will very likely cause the slurry to freeze solid, preventing the pump from dispensing food. High salinity saltwater at 1.026–1.030 is required for reliable operation at the snowflake setting.

We recommend diluting with high salinity saltwater (1.026–1.030 SG) for all saltwater setups. For freshwater setups, use RO water and set the temperature to setting 2 or higher. See our Freshwater Compatibility Guide for more detail.

Why a slurry — not just frozen cubes

Frozen food cubes cannot be dispensed directly by the aF4's pump system. The peristaltic pump requires a pourable liquid slurry to draw food from the reservoir, push it through the solenoid, and deliver it to the aquarium. The combination of the aF4's cooling system and a saltwater slurry creates the right conditions — cold enough to preserve freshness, but fluid enough for reliable dispensing.

This is also why the reservoir has a max fill line — the correct ratio of food to dilution water produces a slurry consistency that the pump can handle reliably. Too little dilution water results in a paste that the pump struggles to draw. Too much dilution water reduces the amount of food delivered per feed cycle.

Related articles

• Loading the Reservoir — how to prepare and load your food correctly
• Temperature Settings Guide — choosing the right temperature for your setup
• Food Freshness & Storage Duration Guide — how long food stays fresh and how to maximise it
• Freshwater Compatibility Guide — RSW adaptations for freshwater setups

Questions about how the aF4 works? Submit a support ticket and our team will help.