Why do particles keep showing after ParticleSystem.Stop()?

Stop() stops emission by default and lets existing particles finish. Use ParticleSystemStopBehavior.StopEmittingAndClear to remove them immediately.

Why does my particle Stop Action never run?

The system is looping and never stops on its own, or child systems still have living particles. Stop Action runs only after the system stops and every particle has died.

Which Stop Action should I use with object pooling?

Disable, or Callback to return the object to the pool. Destroy breaks pooling.

Fix: Unity ParticleSystem Stop() vs Stop Action

Quick answer: ParticleSystem.Stop() stops emitting by default; live particles finish their lifetime. Use Stop(true, ParticleSystemStopBehavior.StopEmittingAndClear) to remove them instantly. The Stop Action (Disable, Destroy, Callback) runs only once the system has stopped and every particle has died — which never happens for a looping system you never stop.

Two separate features share the word "stop": the Stop() method, which controls emission, and the Main module's Stop Action, which decides what happens to the GameObject afterwards. Most particle clean-up bugs come from expecting one to do the other's job.

Stop() and its behaviours

ps.Stop();   // = Stop(true, ParticleSystemStopBehavior.StopEmitting)

StopEmitting — no new particles; existing ones live out their lifetime. Smoke trails fade naturally.
StopEmittingAndClear — stop and remove all particles immediately.

The first argument, withChildren, also stops child systems. Call it on the root of a multi-part effect; stopping only a child leaves the others running.

Stop Action

In the Main module, Stop Action runs after the system is stopped and its last particle has died:

None — nothing.
Disable — deactivates the GameObject. Best for pooled effects.
Destroy — destroys the GameObject. Fine for one-off effects, wrong with pooling.
Callback — sends OnParticleSystemStopped() to scripts on the same GameObject.

void OnParticleSystemStopped()
{
    pool.Release(gameObject);
}

"My effect is never cleaned up"

Looping is on. A looping system never stops by itself, so the Stop Action never runs. Turn Looping off for one-shot effects, or call Stop() when the effect should end.
The callback script is on a different object. OnParticleSystemStopped is only sent to the GameObject that has the ParticleSystem.
A child system is still alive. The parent waits for its own particles; children with longer lifetimes keep their own objects alive. Give every system in the effect the same Stop Action, or manage clean-up from the root.

"My particles vanish instantly"

You called Stop(true, StopEmittingAndClear), Clear(), or deactivated the GameObject directly with SetActive(false) — which hides all particles at once. To let them fade, call Stop() and let the Disable stop action deactivate the object afterwards.

Pooling pattern

public void Play(Vector3 pos)
{
    transform.position = pos;
    gameObject.SetActive(true);
    ps.Clear(true);
    ps.Play(true);
}
// Main module: Looping off, Stop Action = Disable (or Callback to return to the pool)

Clearing before playing removes particles left over from the previous use.

Checking state

ps.isEmitting tells you whether it is still spawning; ps.IsAlive(true) whether any particles (including children) are still alive; ps.isStopped whether it is fully stopped. Waiting for !ps.IsAlive(true) is the code equivalent of a stop action.