For an underfill, the most likely culprit is the safety time out, keep in mind that the maximum size set up a Nano Osmolator can handle is 55 gallons total volume. By default, 25 gallons is the limit, for 25-55 gallons you must open the controller as shown on page 26 and change the jumper to a safety time out of 3 minutes. By default this is 1.8 minutes. The pump can deliver about 1/3 gallon in 1.8 minutes and about 2/3 gallon in 3, after this time is up the pump will pulse off and on as a warning that the time has been exceded and it is shutting down, you then have to reset it. If a tank has a volume of over 55 gallons or a very high evaporation, only the full size 3155 will be able to handle the top off.
An overfill is most likely to be caused by a siphon, if you are pumping to the sump and the water level in the reservoir is higher than the end point of the hose, for example reservoir holds 18" of water and is next to a sump 16" high and the hose hangs into the sump 2", then 4" of water can siphon in after the pump turns off. The best solution is to pump the top off water to the tank and not the sump so gravity prevents a siphon, you must also be sure the top off hose does not touch the waters surface or water can backsiphon from the tank to the reservoir.
The only technical issues we have encountered center around the float and magnet holder. The float is a magnetic switch, the black ring in the float is a magnet and it actuates a switch in the post. It is possible when the glass is very thin, less than 1/8", that the magnet holder pulls on the float, this will be obvious as the float will be angled and not travel straight up and down. Because of this it catches and does not work smoothly, the magnet holder itself can hold the float in place. The solution is generally to add an 1/8" thick shim of rubber or plastic between the outside magnet and the glass.
