In the "Climate Bathtub" model, the level of the water in the tub does not change when the flows to and from the tub are in equilibrium - the same amount of water is flowing out the drain as is pouring in from the faucet.
If we turn up the flow rate from the faucet, more water will enter the tub than is leaving via the drain. The tub will gradually fill with water. This corresponds to a situation in Earth's carbon cycle when more carbon dioxide is being released into the atmosphere than is being removed. In such situations, the amount of carbon dioxide in the atmosphere will increase. In recent times, emissions of carbon dioxide from the burning of fossil fuels and other human activities has created just such a situation. Of course, throughout the long history of our planet, there have been numerous times when natural imbalances between carbon dioxide emissions and CO2 absorption have also created a "filling bathtub" situation such as this.
The opposite situation can also occur. If more carbon dioxide is being absorbed from the atmosphere than is being emitted into the atmosphere, the situation is analogous to a bathtub in which more water is flowing out of the drain than is pouring in from the faucet. In such a case, the water level in the tub goes down. This corresponds to a reduction in the amount of CO2 in the atmosphere.