This weekend, after a few weeks of research, I built an automatic top off system for our 110 gallon saltwater aquarium. Works like a charm. My design is for a sump, but can be altered easily enough should you want to do it for your display tank.

This design allows me to supply a reservoir of fresh water for the aquarium and use it to top off the system as evaporation occurs. I lose about two quarts a day to evaporation and replacing it by hand every night is tiresome.

Here are the details for anyone so interested.

Material
Float switch
(I used one purchased from floatswitches.net, but you can also get them from Grainger.)

Rubbermaid Container
(A ten gallon container to be used as the reservoir.)

Powerhead
(Any flow type will do, but if you use a small enough one you won't have to worry about splashing or fast water movement.)

Extension cord
(A regular extension cord you can find in hardware stores. Make sure it has two distinct wires running down its length, as you're going to splice one and leave one intact.)

PVC piping
(I used a six foot 1/2" outer diameter piece from the Home Depot. If you can get them to cut it for you, go for it. I only ended up needing about six inches worth.)

(2) PVC elbow connectors

(1) Threaded end for the PVC, fit to the size pipe you use.

Flexible tubing, 1/2" outer diameter
(This is either sold in precut lengths or cut to order. I used the precut so I had extra if needed.)

Plastic bulkhead
(This piece will connect the inside tubing of the reservoir to the outside tubing. When properly caulked it forms a waterproof seal.)

Electrical tape

Zip ties and self-adhesive zip tie holders

I won't do a step by step, but can detail one if anyone is interested. Here's a short narrative about it.

I placed the Rubbermaid container next to the aquarium stand and behind a few items to make sure it was accessible, but not visible. A powerhead sits in it and supplies the water flow. The powerhead is attached to the extension cord which has the float switch spliced in line with it.

The float switch will, of course, float on the water's surface. I built an inverted "U" from the PVC pipe and the elbow connectors and secured the switch to the side of the inverted U that's in the sump and just at the water line; the cord runs through the pipe to avoid splashes. It's secured to the sump by a few zip ties and self adhesive holders.

I pierced a tiny hole in the reservoir just below the lid line and used a plastic bulkhead to connect a length of tubing internally and a longer length of tubing externally. Internally, the tubing is connected on one end to the bulkhead and the other end to the powerhead. The powerhead has suction cup feet to keep it secured to the bottom.

The other end of the bulkhead is secured with a drop or two of waterproof caulk and once set, is there to stay. A long length of tubing goes from the external portion of the bulkhead to the sump. I secured mine by running it through an existing hole in the sump, but it's just as easy to create one yourself or use some of the zip ties.

The float switch is in the 'normally open' mode and only closes the circuit when the water drops due to evaporation. Once the water line gets past a certain threshold, the float switch completes the circuit, supplies power to the powerhead and water is pumped into the sump. Once the water level is high enough, the float switch re-opens the circuit and the powerhead turns off.

I set it up on Friday evening and every time I've checked it since then (about a dozen or so times) the water level has been exactly the same. Normally, I have to make one or two trips to the sink with a container to replace the evaporated water each day. I'll probably get a bigger reservoir in the near future, but it's looking like I'll only need to re-fill it once every three or four days.

Now, on to enjoying the aquarium instead of making trips to the sink. Aquarium web cam shouldn't be too far off.

Update: Additional Float Switch