Dealing with evaporation is a critical component of tank stability and protecting your equipment. Here is our take on the various methods of replenishing freshwater lost to evaporation then a complete install of the automatic top off system on our BRS160.

Every aquarium is going to evaporate water, but this is especially severe in a reef aquarium. Reef tanks typically have substantial surface area with considerable amounts of internal water flow and surface agitation and most often don't have a tight lid.

Reef Tank

It is common for average sized reef tanks to evaporate anywhere from one to four gallons a day.

The effects of evaporation are primarily with stability because as water evaporates, H2O molecules leave the tank but the salt and all of the other elements and dissolved compounds in your tank water stays behind. The concentration of these elements and nutrients will rise and parameters will shift. Once you add freshwater back into the tank to account for evaporation, these parameters will dilute back out again causing a fluctuating environment. Certainly NOT ideal for a captive reef tank and its inhabitants.


Outside of that, fluctuating water levels will also put your equipment and home at risk. Heaters, pumps and powerheads, protein skimmers, probes and various other equipment are designed to be operated submerged underwater. As water evaporates, the water level will drop and this equipment will malfunction if left exposed to air for too long. This can also create a dangerous fire and flood hazard in your home because without water, this equipment can melt through your sump or short out and ignite.

Manual Top Off Methods

Topping off manually - economical but cumbersome

There are a few basic ways to top-off your tank. The most economical would be topping off by hand or manually adding fresh RO/DI water, everyday. Most will just pour the water directly into the tank but you can also set up a slow drip to gradually drip freshwater into your tank via gravity and a ¼” tube with a valve. Although this method is sufficient in terms of stability, it is certainly a hassle and if you forget to fill it or want to get away for a few days, it can complicate things.

Tunze Osmolator

ATO or automatic top off system - reliable and easy to implement

Thankfully a better solution exists called an ATO or automatic top off system which is the preferred method of top off by modern hobbyists. An ATO system uses float, optical, heat or electrical sensors to monitor or detect water level and then refill the tank.

Float Valve Gravity Top Off

The simplest form of an ATO is just a reservoir connected to a float valve in the sump. When water evaporates, the float valve drops and allows water to enter the tank via gravity until the level is correct again.

While a mechanical float valve works fairly well in the short term, the seal on the float can easily be fouled by algae, salt creep, a rogue snail or any number of things. When that happens, the entire freshwater container will be emptied into your tank causing a dangerous swing in salinity and possibly overflow your tank or sump causing a flood.

ATO Systems

A better and the most popular solution is a pump driven auto top off which uses float, optical, electrical or temperature based sensors to monitor the water level and a small water pump to deliver water to the tank. The reliability of these pump driven ATO systems is tied very closely to the quality of sensors and the technology behind it.

Float Valve

Float switches have moving parts that can get stuck with algae, salt creep or a pesky snail. Electrical sensors can get calcium deposits or other tank debris stuck on them which will prevent proper function. Don’t let this scare you though because there are a number of trustworthy ATO systems and common practices that will help you avoid these failures.

When selecting an ATO, like most everything in this world, you get what you pay for. Be sure to read reviews and purchase something that is time tested and trusted by hobbyists. Stay away from the “NEW” technology and let others test them. Perhaps the most critical piece of advice is to take note of any reported failures. No one wants the ATO to stop working or break, but what should concern you the most is an ATO system that fails in the ON position where the pump fails to shut off and empties your entire reservoir.

JBJ ATO System

For a reefer on a budget, the JBJ A.T.O. Water Level Controller is the best value. It does rely on float switches which means moving parts but it also has some excellent failsafe features built in. The primary float sensor works in conjunction with a secondary float along with an internal timer which triggers if the pump has been on for too long.

Float Valve Back Up

A smart addition to this style of ATO system is the addition of a mechanical float valve installed on the side of the sump slightly higher than you want the water level. This way it works as a back up but stays out of the water and reduces the chances of something interfering with the seal.

Tunze Osmolator

The Tunze Osmolator Universal 3155 is our favorite and most trusted ATO of all time and for a few good reasons. The primary sensor is optical and doesn't use any moving parts which is much more ideal for a saltwater environment. It also includes a float switch as a back-up sensor along with an internal timer which turns the pump off it it has been on to long as a final failsafe.

Tunze Osmolator Sensor

We have yet to witness an Osmolator fail in the ON position. The sensor can get dirty if you never clean it but this would fail OFF which is much less of a concern. The time tested and proven reliability in both the mechanics of how it works and the quality components results in a ton of online reviews which will further confirm you are getting a top-quality ATO.

Tunze Osmolator Flow Adjustment

In addition to the reliable operation and quality components, the osmolator also includes a pump with variable flow rate that is adjusted via the internal dial. Smaller tanks will require a slower fill rate while larger tanks need to be topped-off faster because of the increased evaporation rate.

Tunze Osmolator Accessories

Tunze also offers a couple of cool accessories making the unit even more versatile. The Water Valve 8555.200 is a ¼” water safe solenoid which can replace the pump with a valve that opens and closes based on the sensors. There is also a Switch Socket Outlet which allows you to use most any AC water pump which is nice for really large tanks because you can attach a much stronger water pump for quicker fill rate.

ATO Reservoirs

With any ATO system, a storage container or reservoir is required to hold the freshwater that is being delivered into your tank periodically; any reef safe container will do. Many hobbyists will hide the container under a stool or inside a chest next to the aquarium in order to save room under the tank because you really want to get something large enough to hold enough water for one week of top off. This saves you the hassle of frequent refill.

Buckets of water

Topping off directly from your RO/DI System - Expensive but permanent and hassle free!

This leads us to the last method of top off which eliminates the need for hauling around buckets of water and filling reservoirs altogether which is hooking your tank directly to your RO/DI system for automatic freshwater replenishment.

A critical aspect to understand in this case is that your plumbing a never ending water supply to your tank which means if it fails somehow, it is almost certainly going to wipe out your tank, cause a flood and destroy your floors or home.

Float valves gets stuck, solenoids can break, you cat or dog might chew through the lines. If you choose this route, just know that you are creating a risk and everything you do is going to be managing that risk.

RO/DI Top Off System

It starts by connecting your RO/DI system to your sump with a float valve. The first step of managing the flood risk is avoid an adjustable float valve and use a quality fixed position float valve. The wing nut used on adjustable floats are too easily bumped which can lead to failure.


Since a float valve in a saltwater environment is not all that reliable on its own, you next want to add some redundancies. A water safe solenoid, connected to a an auto top off system or float switch on an aquarium controller is ideal. In this case, if the water level gets too high in your sump, the sensor will trigger the solenoid to close and shut off the water supply.

Leak Detectors

A stand alone leak detector or leak detector for your aquarium controller like the Neptune Systems Apex can also be used to trigger the solenoid. Keep in mind you can use these solenoids to either shut off water supply to the sump or alternatively to shut off water supply to the RO/DI system itself.

There are also water sensing leak alarms like The Watchdog which sets off an ear piercing alarm if the water level gets too high. For those of you with pets or young children that can tamper with the RO line, consider routing it through a pipe or cable management system to protect it.

BRS Color Changing DI Resin

Something to note if using an RO/DI system for direct top off is you will experience increased DI resin consumption because of something called TDS creep and the frequent on/off type operation. The initial water that comes out of the system once it is turned on is typically higher in TDS. This is just a reality that you will have to live with but considering all of the time you are saving from not having to lug heavy containers of water around the house and spilling water, it is well worth it.

RO/DI Top Off System with wall timer

One clever solution to help reduce the DI resin consumption is using a timer and a solenoid so the RO system only operates during one time period per day. This will have a very minimal impact on stability but will reduce the effects of TDS creep.

You can also build an elaborate system that refills a freshwater reservoir only after it has been emptied but that is likely way beyond what most of us are willing to do in order to save a few dollars on DI resin each month.

Cleaning ATO Sensors

Our last bit of advice for ATO systems, if you don't maintain it, expect it to fail. As part of your monthly maintenance be sure to disassemble and clean all the sensors, valves and pump. It is also a good idea to test switches. If you do this properly, you will get the longest life out of your ATO and can count on reliable performance day in and day out.

ATO Installation on #BRS160

For the BRS160 we want an ultra stable option that requires the least amount of work so we decided to attach our BRS 6 Stage Deluxe Plus RO/DI System directly to the sump. The primary level sensor is going to be the Tunze Osmolator with the solenoid accessory installed inline between our RO/DI system and the sump. Tunze includes a magnetic sensor mount which will allow us to easily adjust the exact water level.

Sensor and Float position on BRS160

To back that up, we drilled a hole near the top of the sump and installed a fixed float valve. We also installed a Flow Lok leak detector outside of the sump. The Flow Lok is a neat RO accessory which has a pad that expands when it gets wet and triggers an inline shut off valve.

Flow Lok and Protective PVC Pipe for RO Tubing

We used some PVC pipe attached to the wall with clamps to protect the RO tubing so no animals, humans or anything else can damage the RO tube. Even with all this, the ATO system could still fail but we have done our due diligence and the benefits outweigh the risks at this point.

A controller such as the Neptune Systems Apex with a leak detector and float switches would be the final failsafe to this system. With the ability to not only control a solenoid valve but also send alerts via email or sms, you should never have a worry or wonder what is going on with your tank when your away.