Top-up cap is a simple adapter for plastic bottles that turns it into a system for topping up evaporated water.

Top-up cap screws onto the neck cap filter of the bottle after filling it with osmosis water or even water enriched with supplements.

The cap has two small tubes, which allow the minimum level to be adjusted for replenishment. The working principle is very simple, if both tubes are in the air, the water can go down into the aquarium. If the longer length tube (trigger tube) is submerged the air cannot enter the bottle and water cannot go out. the system is in stand-by.

For proper operation, I recommend cutting the primer tube at 45° instead of horizontally; in order to minimize the noise of bubbles during refilling.

Top-up cap is completed with 3 adapters that change the thickness of the glass to which it can be applied; 4, 6, 8 and 10mm are the allowed glass thicknesses.

The bottle filled with water has a fair amount of weight, make sure it is placed firmly and securely even from possible accidental blows by placing it close to the wall, if any.

NB: the tubes should not be cut equal, operation would not be possible; one of the two tubes must be longer in order to stay below the water level while the 'other tube lets air through.