Lily pipe placement: the geometry of whole-tank CO₂ distribution

The difference between a well-placed and a poorly-placed lily pipe can be the difference between consistent plant growth across an entire aquascape and a tank where one corner always lags. Understanding why requires a small amount of fluid dynamics — but the practical rules that follow are simple.

The Coanda effect

When a fluid jet flows close to and parallel to a solid surface, it tends to attach to that surface and follow its contour rather than expanding freely into open space. This is the Coanda effect — a well-documented phenomenon in fluid mechanics, observed across liquids and gases alike.

In a planted aquarium: a lily pipe outlet positioned 2–4 cm from the back glass, aimed along that wall, produces a jet that hugs the glass and sweeps its entire length. The same outlet positioned 10–15 cm away produces a jet that expands into open water, travels a short distance, and dissipates — covering perhaps a quarter of what a wall-attached jet would reach.

The back wall is where you want the initial jet to travel, because it forms the first leg of the whole-tank circulation cell — the gyre.

Building a gyre

A gyre is a large-scale, coherent circulation cell. In a rectangular aquarium, a correctly placed lily pipe creates exactly one: the surface jet sweeps along the back wall to the far corner, descends down the right side wall, travels back along the substrate, and rises up the left side wall to return to the lily pipe. One continuous loop — the same water column visiting every corner.

This matters for CO₂ distribution because CO₂ is consumed at the leaf surface, not in the open water column. A single pass of CO₂-rich water is not enough. What you need is continuous circulation that constantly refreshes the boundary layer immediately around each plant. A healthy gyre does this simultaneously across the whole tank.

Animated flow: what each placement actually does

The four panels below show how water circulates — or fails to — under different outlet positions and angles. Particles represent water movement. Watch how a back-corner outlet fills the entire tank volume, while a centre or surface-aimed outlet leaves large areas completely stagnant.

The five placement rules

These rules follow directly from the fluid dynamics above. They apply equally to ADA glass lily pipes, ceramic versions, and standard plastic filter return outlets.

1. Back corner, not centre. A centre-back position divides flow into two weak half-tank gyres that produce dead zones in all four corners. A back corner creates one large gyre covering the whole tank.
2. 2–4 cm from the back glass. Close enough to activate the Coanda effect, far enough that glass lily pipes don't vibrate against the wall.
3. Tip 1–2 cm below the waterline. Deep enough to create surface movement without breaking the surface. If the outlet tip is at or above the waterline, it creates surface turbulence and direct CO₂ loss.
4. Aimed parallel to the back wall, not into it. The outlet should fire along the glass, not at a right angle to it. A slight downward angle of 5–10° helps the jet reach the far end before it rises.
5. Fire toward the longer dimension. In most tanks this means left-to-right (or its mirror). The longer the jet travels before hitting the far wall and descending, the more of the tank volume the gyre encompasses.

What to look for at the surface

A correctly placed lily pipe creates a thin, fast-moving film of water across the surface — visible as a gentle shimmer when you look across the waterline from the side. This surface movement provides adequate gas exchange to maintain dissolved oxygen, and prevents the stagnant surface film that would otherwise reduce CO₂ absorption into the water column.

What you do not want is visible surface rippling — waves, splashing, or broken water. This indicates the outlet is too close to the surface, angled upward, or running at a flow rate too high for the outlet size. Each accelerates CO₂ off-gassing significantly.

If you are logging CO₂ stability with a pH monitor, poor lily pipe placement often shows up as an inconsistent mid-session curve — CO₂ peaks and then drops irregularly rather than holding steady. This frequently indicates localised turbulence at the outlet causing CO₂ loss faster than the diffuser replaces it. Repositioning the outlet often smooths the curve significantly. See the guide to CO₂ stability for how to read and act on these curves, and the pH monitor guide for how to log them.

Spray bars: a different approach

Spray bars distribute flow along their length rather than concentrating it in a single jet. This makes them inherently better at covering width, but they don't drive a gyre in the same way — the distributed flow tends to produce a broad sheet rather than a concentrated circulation cell.

For spray bars, the common recommendation is: mount along the back wall or one side wall, angled downward at 30–45° toward the substrate. This creates a sheet of downward flow that pushes water along the bottom and generates a return current up the opposite wall. It's a weaker version of the gyre effect, but adequate for many tanks.

Spray bars aimed toward the surface are among the most aggressive CO₂ off-gassers in common use. If yours is pointed at or near the waterline, expect to lose a meaningful fraction of your CO₂ before it reaches the plants.

Scaling for tank size

• Under 80 litres: a single back-corner lily pipe covering the full length of the tank is sufficient. The gyre is compact enough to cover the whole volume in one pass.
• 80–200 litres: back-corner placement still works well. Verify your filter provides at least 5–8× turnover — even a perfectly aimed outlet with too little flow will leave slow spots.
• 200+ litres or very long tanks: consider two outlets in opposite back corners, each firing toward the centre of the tank. This produces a figure-eight circulation that covers both halves without the two jets opposing each other.

For the bigger picture on flow rates — how many times per hour the tank volume should turn over, and why that number matters for plant growth — see the guide to flow in a planted tank.