A wallpaper is not really a picture you look at. It is a picture you look past all day, while you are actually checking the time or reading a notification.
That changes what makes an aquarium scene good. A reef photo that looks rich on a laptop can become a field of speckles behind your clock. A jellyfish drifting through dark water sounds dull described in words, and tends to still be there a month later.
So if you are choosing between reef, jellyfish, koi and deep sea, here is what separates them on a phone screen.
The screen you are actually filling
An iPhone screen is a tall, narrow rectangle, and most underwater footage was shot wide. Crop a wide reef to portrait and you keep a vertical slice of coral while losing the sweep that made the shot work.
It gets narrower than that. The top of the lock screen carries the date, clock and any widgets. The bottom carries two control buttons and the home indicator, though on iOS 18 and later you can change or remove those. Notifications fill the middle band through the day. What you see uninterrupted is two thin strips, so a scene whose best moment sits dead centre is one you rarely get to see.
Reef scenes look best in a listing and worst in use
Reefs are the obvious choice: coral, anemones, colour, small fish everywhere. The problem is that a reef has high contrast in every square inch. Every polyp is an edge, so white text on top has nothing to rest against, and iOS draws no shaded plate behind the lock screen clock to rescue it.
If you want a reef, find one with a clear column of open water above it, so the coral occupies the lower half and the top stays quiet. That single choice does more for readability than any colour adjustment.
Jellyfish move slowly and are shaped simply
Jellyfish scenes work for two reasons that have nothing to do with taste. A bell is a large, simple shape, and large simple shapes survive being scaled down to a phone. Fine texture does not. And the motion is a slow pulse rather than a dart, so the edge of your vision files it as ambient rather than as something that needs a look.
If you want motion you notice when you choose to and ignore the rest of the time, this is the safest of the four.
Koi work because you are looking down at them
Koi scenes are usually framed from above the water. That gives you a large, near-uniform dark background with a few bright bodies crossing it, which is close to the ideal lock screen: strong contrast where the subject is, almost none anywhere else. Koi markings help too, being big blocks of white, orange and black rather than fine detail.
Watch the water surface. Heavy ripple produces a shimmer of small bright highlights, and if that shimmer sits under your notification stack it makes every banner slightly harder to read.
Deep sea is the quietest option, and the kindest to an OLED
A deep sea scene is mostly black with a few lit shapes moving through it, and two things follow. White text gets the cleanest background it can get. And on an OLED iPhone, which is most models since the iPhone X apart from LCD ones such as iPhone XR, iPhone 11 and the older SE, black pixels are simply not lit, so a mostly black wallpaper asks less of the battery than a bright one at the same brightness. On an LCD model the backlight stays on either way.
Motion scale matters more than fish count
The useful measure is not how many fish are in the scene. It is how far anything travels per second and how sharply it changes direction. Peripheral vision responds to sudden movement and to changes of speed, so one fish crossing the full width pulls harder than a shoal drifting in shallow arcs.
Look for nothing crossing the whole screen quickly, no hard turns, and no object that visibly snaps back to the other edge when the animation loops. That last flaw is the hardest to stop seeing. Aquarium 3D: Live Wallpapers runs each scene inside a phone-shaped frame before you save, which catches an awkward swim path faster than saving and starting over.
Judge it with your notifications on screen
A wallpaper that looks calm in a picker can fall apart in use. Once you set it, wait until a few notifications have stacked on your lock screen, then look at it in a dark room at the brightness you use at night. That is the condition you see it in most often, and where a shimmering water surface or a bright fish crossing the middle band becomes a real problem. If it is hard to read then, change the scene rather than the clock font.
Quick answers
Do aquarium wallpapers drain the battery?
The wallpaper is a still image most of the time, so the ongoing cost is small. What costs power is screen brightness and how long the display stays on. A dark scene on an OLED iPhone does use less than a bright one, but that difference is smaller than how often you wake the screen.
Will the fish move on my home screen?
No. The iPhone home screen has never played animated wallpaper. Any motion lives on the lock screen, and even there it depends on your iOS version and the wallpaper type. Pick the home screen image for how icons sit on it.
Which scene suits a lock screen full of widgets?
Deep sea or koi. Both give large areas of near-uniform dark water, so widget panels and their text stay readable. Reef scenes are the worst match, because the fine detail and the panels compete for the same space.
Can I use a different aquarium for each Focus?
Yes. Each lock screen can be linked to a Focus, so Work switches you to a plain deep sea scene while your evening Focus brings back the reef. Touch and hold the lock screen with the phone unlocked, swipe to the one you want, then tap the Focus button underneath.
Where to get it
If you would rather not hunt through video sites for footage that was never framed for a phone, Aquarium 3D: Live Wallpapers has reef, jellyfish, koi and deep sea scenes built in portrait, with a preview that runs the motion in a phone frame so you can check a swim path against your own clock position.
Pick one scene, live with it for a week, and change it only if you catch yourself looking at it instead of past it.