Containers: Open vs. Sealed
The choice that frames everything else
Before you pick water, soil, or species, you choose a container: is the top open to the room, or sealed shut? This one decision changes how the whole ecosystem behaves, because it decides whether your tank can breathe.
An open tank exchanges gas freely with the atmosphere. Oxygen the air holds in effectively unlimited supply is always available at the surface; excess CO₂ escapes; the room is an infinite buffer. This is the normal aquarium case.
A sealed tank — a closed jar, a bottled ecosphere, a lidded experiment — is a closed world. Nothing crosses the glass. The only oxygen the inhabitants will ever have is what the water holds plus what sits in the air gap above it. The only thing that removes CO₂ is photosynthesis. Over a night, if the animals and bacteria breathe faster than the plants and algae can photosynthesize, there is no atmosphere to rescue them. A sealed tank that thrives is one where production and respiration balance over each 24-hour cycle on their own.
The physics of how gases cross the water surface and partition into the air gap live in Dissolved Gases and its gas-exchange section. This page covers the container choice — what it means and what you set.
The headspace is the oxygen buffer
The headspace is the gas-filled gap above the waterline. In an open tank it barely matters — it's continuous with the room. In a sealed tank it is the single most important number in the whole setup: it is the entire overnight oxygen reservoir.
Picture a planted jar at dusk with a full day's oxygen dissolved in the water and stored in the air gap. Through the night, respiration draws that reservoir down. A jar with a generous air gap coasts through to morning; a jar filled almost to the brim has almost no buffer and can suffocate before the lights — or the sun — come back. The same air gap governs how fast the dramatic sealed-tank gases build up: methane bubbles (ebullition) collecting at the top, hydrogen sulfide from anaerobic sediment, ammonia off-gassing at high pH.
Fill level sets the headspace
For a sealed tank you set a water fill level — how full the tank is. The unfilled remainder above the waterline is the headspace:
headspace volume = tank footprint × (height above substrate) × (1 − fill level)
Fill to 90% and a tall slice of air sits on top as the buffer; fill to 97% and you've left almost nothing. A sealed tank cannot be filled to 100% — an airtight, completely full container has no gas phase at all, so the simulator requires the fill level to leave a real air gap (and not be so empty the water column is unrealistic). Lowering the fill level both enlarges the headspace and reduces the water volume, which concentrates the biomass per litre — a denser tank with a bigger buffer.
Open tanks don't expose this control: they're treated as filled to the brim, and their (vented) headspace doesn't change the outcome because the room resupplies whatever the water needs.
A sealed tank runs without maintenance
A sealed ecosystem is a no-intervention system by definition. You can't change its water, dose fertilizer, or trim plants without opening it — and opening it breaks the seal you chose. So for sealed tanks the simulator skips the whole Maintenance step: no water changes, no liquid-fertilizer dosing, no floating-plant trimming. The tank runs closed from day 0 to the end of the simulation, and whatever nutrients and organisms you started with are all it will ever have.
This is the point of a sealed tank, and it's what makes it a real test of ecological balance: every gram of nitrogen, every cycle of carbon, has to close on its own. Open tanks keep the Maintenance step, where water changes, dosing schedules, and trimming model the hands-on care a normal aquarium receives.
What you're actually setting
| Container | Top | Headspace | Maintenance | Typical use |
|---|---|---|---|---|
| Open | Exchanges with the room | Vented; size irrelevant | Water changes, dosing, trimming available | Normal planted / livestock aquarium |
| Sealed | Airtight | Set by fill level; the overnight O₂ buffer | None — runs closed | Closed jar, bottled ecosphere, balance experiments |
Under the hood the container selection sets the gas-exchange physics: an open tank lets the headspace relax to atmospheric composition; a sealed tank conserves total oxygen and carbon, redistributing them between water and air gap but never gaining or losing any. See Sealed vs. Open Systems for the full treatment.