One of the issues a saltwater aquarist has to contend with is loss of water through evaporation. As most marine systems employ much higher wattage lighting than freshwater tanks, the rate of loss can be surprising. As an example, my 135 gallon tank can lose up to 1.5 gallons a day in the summer!

Another issue that especially plagues smaller tanks is the change in salinity caused by this loss. As only pure water is lost, the salt remains in the tank, and salinity increases. The loss of 1 gallon of water in a 15 gallon nano reef can shift the specific gravity (SG) from 1.024 to 1.027. This can be stressful on corals and livestock alike.

My (very) low tech approach to this is to use the same float valve that is used within reverse osmosis systems. I purchased this specific float valve from eBay for $9 plus shipping.

Materials Needed

¼" air-line tubing ($10)

¼" valve (($1.25)

Scrap plexi-glass/acrylic/plastic (free?)

Float valve ($9)

Nylon hardware ($1.50)

Water container/ 5 gallon bucket/etc. ($3-$15)

Tools needed

Drill

5/8" drill bit

¼" drill bit

superglue

The first step is to remove the metal hardware in the float valve and replace it with nylon. Drill out the old screw with a 1/8" bit. Thread the new hardware in and tighten it down. You could also simply glue the pieces together instead, and lose the ability to adjust the level. I purchased the nylon hardware at my local Home Depot for roughly $1.50

You then take a scrap of acrylic/plexiglass/plastic and cut it to roughly 2"x4" in size. This is then bent into a 90 degree angle using a heat gun. The plastic can be any sufficiently robust material available. The heat gun can be replaced with a hair-dryer as well, assuming you have patience. The hair dryer adds considerable time to the job.

You then drill out a 5/8" hole into a 5 gallon bucket, and plumb it with a ¼" line quick release fitting. This will be the RO/DI water reservoir. It could be substituted for a 5 gallon water carrier easily, especially because they have a nipple available to use for this purpose.

5 gallon bucket with plumbed quick release fitting





5 gallon water jug with threaded fitting on top of pic





The float valve is then inserted into the drilled hole and screwed down tightly. Insert about 8" of ¼" line into the float valve and then a ¼" valve. Then apply as small quantity of superglue to the vertical face of the plastic (it helps if it is scuffed up somewhat). The angle plastic is then placed at the correct height inside the sump.

Run the ¼" tubing back to where you placed your reservoir and cut it to length. Fill the reservoir at this point, and find the free end of the tubing. Suck on the tubing until all the air has been displaced, then connect it to the check valve. Turn on the check valve at this point and you’re done!

If you are a belt and suspenders sort of person you can add another float valve into the reservoir itself. This valve will be installed upside down, and will use gravity to close the valve. This will prevent the line to the sump from sucking air into it if the reservoir is empty. Not really worth the extra effort in my opinion, but if you are likely to let the reservoir run dry it’s not a bad idea.

Images of this setup in operation below.

The float valve glued in place









The ball valve to shut off flow









Wider angle shot of float valve in operation (note the “max fill” marking)