Aquarium Auto Top Off Systems

An auto top off (ATO) is one of the most time saving and cost effective automation systems the aqurium hobbyist can employ in their fish tank.  The uses for an ATO vary from simply replacing water as it evaporates from the system to providing dosing the aquarium with supplemental materials such as calcium and iodide.  These systems are relatively inexpensive and simple to build and operate.  

There are two main designs Reservoir Auto Top Offs (ATO) and RODI Direct ATO.  Both of these methods accomplish the same goal of replenishing the aquarium system water that evaporates through the course of the day.  The below discussion considers the advantages and disadvantages of each.

Reservoir ATO

Description:   Reservoir ATO systems are designed with a container of fresh water that is periodically filled for the purpose of replenishing evaporated water from the aquarium system.  It can be operated with a gravity feed or a pump feed. 

Advantages:

• System has a limited amount of water that can spill out in the event of a system failure.
• Allows dosing to occur in a controlled manor without the need for sophisticated dosing pumps.
• Can be designed with minimal risk of spill outs.  Easily built with multiple redundant failsafe controls.

Disadvantages:

• Requires space to house reservoir system.
• Requires periodic refills depending on the size of the reservoir.

RODI Direct

Description:   RODI Direct ATO systems are designed to run a line direct from the fresh water source (usually an RODI unit) into the aquarium system at the sump or display and controlled by a float switch. 

Advantages:

• This system does not require space for a reservoir.
• Unlimited water supply.  No periodic refilling required. 

Disadvantages:

• RODI systems function better with a continuous flow.  Using in an ATO system results in short bursts of water moving through the system.
• No opportunity to leverage the system for dosing.
• Potential for unlimited water spill outs and more difficult to build in fail-safes.
• Could be difficult to install due to the requirement for a water line to be run to the aquarium system.

Other Design Considerations: 

• When designing the ATO system, redundant float switches should be used in the event of a switch failure.
• An AquaLifter pump is ideal for use in the ATO system as small amounts of water volume are required with each aquarium low water signal.
• Gravity feed system can be designed using solenoid valves instead of a pump.
• Consider a siphon effect.  Be sure the ATO system does not pose a risk of water siphoning from the ATO reservoir.

Additional resources:

ATO Electrical Diagram