An old iPhone in a drawer is a screen with a battery and a stand mount, and one of the better things to do with it is turn it into a small ambient display. An aquarium scene is a natural fit: it is meant to be glanced at rather than read, it looks right at a distance, and nobody minds it running in the background of a room.
This is a different job from a wallpaper on the phone you carry, and the settings that make it work are different too.
Why a lock screen will not do it
The obvious approach is to set an aquarium wallpaper and leave the phone unlocked on a shelf. It does not work, for two reasons.
The screen turns itself off. Auto-Lock is what governs that, and while you can set it to Never, doing so on a phone you actually use is a bad idea. On a dedicated old device it is fine.
And the lock screen does not animate continuously anyway. Nothing on an iPhone lock screen loops in the background. Motion happens on wake, on unlock and on tilt, then holds. So even with the screen kept on, you get a still picture.
What you want instead is something that keeps playing while the screen is on, which means an app in the foreground rather than a wallpaper behind everything.
The settings that matter
Auto-Lock to Never. Settings, Display and Brightness, Auto-Lock. Only do this on the dedicated device.
Brightness low. An ambient display that is brighter than the room is a lamp rather than a decoration. Turn auto-brightness on and then lower it further by hand.
True Tone on. For once this is genuinely useful. It matches the display's white balance to the room, which is exactly what you want from something meant to sit in a space rather than be looked at directly.
Keep it plugged in. A screen that never sleeps will not last a day on battery, and an old battery less than that.
Guided Access if children are around. Settings, Accessibility, Guided Access locks the device to one app until you enter a passcode.
Watch the heat, not the battery percentage
The thing that shortens an old iPhone's life in this role is not the number of charge cycles, it is sitting at a high temperature while permanently plugged in and running the display. Give it air, keep it out of direct sunlight and off soft surfaces that trap heat, and take the case off. A phone doing this job in a cool spot will keep doing it for years. The same phone wedged behind books on a warm shelf will swell its battery, and that is a repair rather than an inconvenience.
Choosing the scene for a room rather than a pocket
A wallpaper is judged at arm's length in a second. An ambient display is seen across a room, out of the corner of your eye, for hours. The criteria change completely.
Slow motion over fast. Anything with quick movement becomes distracting in peripheral vision, which is the whole failure mode here. Drifting is right, darting is not.
Dark over bright. A bright screen in a dim room is the only thing anyone will look at. Deep water scenes are ideal.
Wide over detailed. Fine detail is invisible at that distance, so it buys nothing and costs sharpness. A simple composition with one or two large shapes reads properly from across the room.
Loop rather than event. A scene that has a moment in it draws attention every time the moment comes round. A continuous ambient scene never does.
Where to put it
Placement decides whether this reads as a considered object or as a phone someone left out, and it costs nothing to get right.
At eye level when seated is the rule. A screen below eye level shows you the sky-facing glare from the ceiling light and nothing else, which is why a phone flat on a coffee table never works. A shelf, a small stand, or a mount at roughly the height of a picture frame all work.
Away from a window is the second rule. Daylight beats any phone screen, so a display next to a bright window is invisible for most of the day and then suddenly the brightest thing in the room after dark.
Against a plain wall is the third. The illusion depends on the screen reading as a window into water, and a busy background behind it breaks that immediately.
And landscape rather than portrait, if the scene allows it. Aquarium compositions are almost always wider than they are tall, and turning the phone on its side gives you the whole scene rather than a cropped column of it.
Which old iPhone is worth using
Almost any of them, with one caveat about the screen type.
An OLED model gives you genuine black, so a dark aquarium scene looks like a window rather than a lit grey panel. An older LCD model has a backlight that is always on, so the dark areas glow slightly and the effect is weaker. Both work, the OLED is noticeably better for this.
Screen size matters less than you would think at a distance. What matters more is placement: at eye level when seated, not on a low table where you look down at it.
Quick answers
Will leaving the screen on damage it?
Modern iPhone displays are durable, but a static bright element left on screen indefinitely is the classic risk on OLED. An animated scene with no fixed interface elements is far safer than a still image with a clock in a fixed position.
Can I use my main phone for this occasionally?
Yes, and it is a good use of a phone on a desk during work. Set Auto-Lock back afterwards, and keep it charging while it runs.
Does it need wifi?
Only to install the app and download the scenes. After that it can run offline, which is convenient for a device you are not otherwise maintaining.
What about an old iPad?
Better, if you have one. Bigger screen, larger battery, and the same settings apply. Scenery reads well at that size.
Where to get it
Aquarium 3D: Live Wallpapers runs its scenes full-screen in the app, which is what makes this work at all, and includes the slow deep-water compositions that suit a room rather than a pocket.
Put it at eye level and turn the brightness down further than feels right. Both make more difference than the scene you pick.