Shower/lavatory + water purifier (closed circulation). These first two objects need clean water in order to work. After using them, they produce the same amount of contaminated water. But instead of continuously supplying clean water and storing its contaminated version somewhere in the base, you can create a self-sufficient circulation of this resource. Over/next to/under the shower and/or toilet room, you can build a room with one water purifier (plus a compactor with sand only) that you should switch off immediately. Connect everything with pipes with a minimum length of several fields and bring in clean water. After several uses, the pipes will begin to transport contaminated water. When you have more than a few fields worth, remove the pipes that bring clean water into the system and turn on the purifier. From now on, the duplicates will use the same water to handle their needs. The system will continue to work as long as you have sand in your stores.
An advanced airlock. The construction of a single airlock by itself is not as effective as the construction of a proper, advanced airlock system. You can achieve this effect in several ways. The first is the construction of a double-doored entryway. Thanks to this, only part of the gas will escape to this room and even less will go beyond the base. The most efficient way, however, is to build a pump system with two vents and a filter. You put the pump in the entryway, which is the room between the base and the outside world (or a room in the base with dangerous gases in it). Next to it, you place a filter that will selectively drain oxygen through one pipe back into your base. The rest of the gases go through the second pipe to the outlet on the outside. This way, the oxygen that will want to escape will return back to the base instead. Before putting up this system, you can build a switch that will allow you to control the situation. Not only does this work, but it also looks great aesthetically.
There is also the so-called "water airlock". Since no liquid allows gases to rise or pass through it, you can build a small tank, e.g. 3 fields wide and 2 fields high. Fill it with water, build ladders and enjoy the ultra-airtight airlock! The downside ... is the tightness, because it will never let the air pass to the other side, even if you need the oxygen. It also requires careful flooding and putting up pipes in its location. Unless, of course, you can find a natural tank and adjust it to your needs.
An air ventilation system versus stationary production. The production of oxygen from algae deoxidizers in one place of the base, or even scattered across different rooms is not very effective. Not only do the duplicates have to run around with resources, but said resources also run out very quickly. In turn, the electrolyzer is a very good machine, but it also produces contaminated oxygen. However, using the electrolyzer, you can build an effective ventilation system.
Build a large room, with several electrolyzers connected to water (depending on the number of colonists), two pumps (one on the floor, the other on the ceiling) with a filter for each, pipes that lead out of the room and two vents inside the room next to each of the pumps. Seal the room completely or build an airlock and set its status to locked. Now set the bottom filter, so that it selects only oxygen and transports it out of the room, and let the main pipe transport the gases to the vent above. The upper filter should be set to only select hydrogen and pump it out of the room (preferably to a generator). The rest of the gases go from the filter to the vent at the bottom of the room. This way the gases will circulate. Oxygen pipes, that emerge from this room should be spread around the base, into different locations, not just one (you are creating a ventilation system, are you not?). With a few vents, you will avoid the problem of having a concentration of gases that is too high. The game will distribute an equal amount of gases to each vent. No human input is required to run the electrolyzers, so once all is set up and functioning, there will be no need for inspections or visits from the duplicates. This system will allow you to supply the whole base with oxygen.
The Morb „Farm". Creatures can be used for specific purposes. The Morb is a great example of recycling of unwanted gases into bearable, contaminated oxygen. Build a closed room and sacrifice the worst duplicate for this purpose. When he dies, a Morb will appear every few days. This way you can breed a small group. Earlier on, you should build an outlet, through which you will pump in unwanted gases. You should also build a pump that will only pump out contaminated oxygen, if you do not have enough pure oxygen in stock.
The Hatch „Farm". This species will be useful when in a larger group. Otherwise, it is an interesting supplement, rather than a permanent source of carbon. This farm will be difficult to create, because sometimes these creatures are not generated on the map. Leave some intact dirt, so that it has somewhere to burrow. Reduce the area, however, by building walls 3 fields high, because it can climb like your duplicates. Place a storage compactor somewhere in the middle and fill it with the mineral you have the most of. Usually it will be sandstone. You should also allow for the storage of coal. When your people already fill the compactor with minerals to some degree, turn off the storage option. The compactor will throw out the materials, and the Hatch will eat them and expel coal at night. This works infinitely (as long as it is not too hot in the room, otherwise it will die).