How to do salt water topoffs | Tropical Fish and Coral

1. Saltwater Tank Top-Off: Replace Evaporation Without Raising Salinity

Reef aquarium with a separate freshwater reservoir and automatic top-off equipment beneath the tank.

A falling water line does not always mean your aquarium needs more saltwater. Evaporation removes water while leaving salt behind. Replacing that loss with saltwater makes the remaining water saltier.

Step 1: Know which water to add

Use fresh RO/DI water for evaporation. Use correctly mixed saltwater to replace saltwater removed during a water change. These are two different jobs.

Step 2: Establish a normal water level

With the system running normally, mark the operating water level. In a sump or all-in-one aquarium, evaporation usually shows in the return-pump chamber. Check salinity with a calibrated instrument rather than judging it by appearance.

Step 3: Choose manual or automatic top-off

Manual top-off requires regular attention. An automatic top-off system, or ATO, uses a sensor and pump to add freshwater when the level falls. Install it according to its manual, including its overflow and siphon protections.

Step 4: Keep the safeguards working

Use a separate, limited reservoir instead of connecting an unlimited water supply directly to the tank. Secure the tubing, inspect the sensors, and refill the reservoir before it runs dry. Disable the ATO during water changes.

Quick tip: An ATO replaces evaporated water; it does not measure salinity or correct an existing salinity problem.

Reference: Bulk Reef Supply: Auto Top-Off FAQs