Auto top-off recommendations

Can someone recommend an inexpensive, reliable auto top-off system for RO/DI water?  I am trying to maintain consistent salinity in my copepod culture. 
 
Thanks,
Chad

Two ways:
1.  Install a float valve on the tub.  Connect to a reservoir located above the tub.  Gravity is your friend.
 
2.  McGuyver alternative: Determine the height of the copepod tub meniscus.  Build a table at the perfect height:the table will hold a dishpan of water containing a plastic gasoline can on its side, opening low on the side.  The height of the top of the hole of the gas can has to be precisely at the meniscus of the copepod tub.  Fix a siphon tubing (airline will do) to connect the dishpan and the copepod tub so that both ends are submerged, and the line is filled with water. The line must go up and over the side of the dishpan.
Close the gas can lid.  Now fill the dishpan up with RO water to the meniscus level while holding the gas can down to estimate its volume displacement. Now fill the gas can all the way to the top with RO water.  Put your hand over the opening to seal it closed, and lay the can so the opening is submerged in the dishpan.  The majority of the can should be above the water line.  Adjust the water line of the dishpan to match the water line of the copepod tub, which should be level with the top of the gas can opening.  
 
Now as water evaporates from the copepod tub, the siphon will draw water from the dishpan.  As the water level in the dishpan drops, the opening to the gas can will be exposed to the air and Glug, air enters can, and an equal volume of water exits the can until the entrance to the can is once again submerged.  Convoluted, but it works and is cheap.
 
I would go with 1.

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Thanks Kathy.