Collection: ATO Systems

What an auto top-off does

Water evaporates from an aquarium; the dissolved salt does not. Left alone the level drops and salinity rises, then falls again sharply when the tank is topped up by hand. An auto top-off adds fresh water as it is lost, so specific gravity stays where it was set instead of cycling. It is usually the first piece of automation added to a marine tank.

How the systems here differ

• Sensing - optical sensors are the most common, often paired with a second sensor or a float valve as backup.
• Delivery - most kits include a small DC pump; solenoid versions instead open a valve plumbed to a reverse osmosis line.
• Standalone or controller-linked - some units run on their own, others connect to an aquarium controller for logging and alerts.
• Sumpless designs - kits built around a hang-on box for aquariums without a sump.
• Liquid level sensors - sold individually in several lengths for reading depth in a sump or reservoir.
• Automatic water changers - units that remove old water and add new on a set interval.

Installing one safely

An auto top-off fails in one direction that matters: it carries on adding water. Mount the sensor away from bubble streams and surface movement, keep the outlet above the water line or fit a siphon break, and fill the reservoir with fresh water only, never saltwater. Size that reservoir to your evaporation rate so refills land on a comfortable interval rather than daily. Where a backup float valve is supplied, fit it rather than leaving it in the box. Email sales@jlaquatics.com to match a system to your tank.