As you know, evaporated water in your reef aquarium or fish only marine aquarium needs to be substituted with fresh water because salt does not evaporate.

I have seen many people use electric float valves directly plugged into their sumps which turns on a water pump when the water drops below a targeted point. In my opinion this is a a problem waiting to happen unless there are several safeties built in, this can be expensive and still not 100% fail proof. If the float valve or pump fails OPEN not only will you flood your floor you will also more than likely harm all your animals due to the fact the salinity levels will drop drastically. If you still insist on using an electric float and pump set-up, I suggest to look any further then the Tunze osmolator auto top-off. I have some friends who use it and they are satisfied.

If you want a Do-it-yourself gravity auto top-off, don't hesitate to keep reading.

I have been operating a gravity feed along with a float set-up for the past 4 years without any issues. If kept really clean (clean algae and causations off the sump float valve) it will in no way over fill your sump with fresh water unless some freak accident occurs.

Here is a checklist of what you will need to build your own auto top-off.

An RO / DI unit (for both top-ups and water changes) a small water reservoir, an auto shut off pressure valve, a ¼" 1 way valve, an optional ¼" solenoid to switch main water to RO on and off, an electronic timer to control the solenoid, two plastic float valves, a bunch of ¼ tube , a 1/4″ bulk head to connect the ¼" tube from the reservoir to the sump valve.

How does it all work? It’s simple actually.

In order to keep my reservoir filled, my RO-DI runs for about 1 hour per day ( via electric solenoid). I do not procure much evaporation so that is a bit more than I need. It fills up the top-off reservoir tank up until the float valve blocks the RO tube causing backpressure, this pressure is what triggers the auto shut-off , therefore turning off the RO unit. It is fairly hard to explain this without pictures. The best way to get your RO-DI all plugged up with the 1 way valve and auto shut-off is to pay a visit to your RO-DI supplier website for instructions. Search for the filter guys, they have some great tutorials and schematics on their website.

I have the 2nd plastic float valve drilled into my sump, the other end connects into my reservoir. The ideal part about this set up is that it is gravity fed, no pumps. The valves work in such a way that they open ever so little as the water evaporates. Which is why the water trickles in at the same rate it evaporates. Salinity is very consistent!

To recap.

There are basically 2 systems in play here. The 1st one is the RO that fills up a reservoir tank that holds your top-off water. The reservoir has a float in it that when closed causes the RO-DI to switch off. On top of that you have an optional electric solenoid controlled by a timer that turns off the water supply to the unit. Overkill? Possibly, but I’d rather not risk my livestock and floors for a $20 switch.

The second is moving the water from the reservoir into the sump when the sump water evaporates. With the set-up stated above you now have filled up your reservoir. In order to get it to top-off you will need to join a ¼" line to a bulkhead drilled into the reservoir tank and the other end of the line into the float drilled into your sump. As the water level of your sump lowers, the float will open slightly allowing water in.

Notice the double fail safe? Truth be told there are doubters but I love this set-up. No extra pumps, it’s secure and affordable.

Happy reefing and enjoy your own reef aquarium auto top-off.