Most doser pumps run about 1.4-1.8 mL per minute which is not fast enough to use for top off - this is literally going in one drip at a time. The other trouble is that tanks don't evaporate the same amount everyday and especially different at different times of year, and doser pumps need to have timers (often built in) to operate them, they don't have a float switch to sense when the water level is low.
Tunze now makes a nano auto top off for around $125. Check into this option.
