Glassfish and Sweepers: The Shimmering Clouds of Red Sea Caves

There's a moment on certain Red Sea dives that stops divers in their tracks: you swim into the shadow of a cave, an overhang, or a wreck's hold, and the space is filled — utterly filled — with thousands of tiny, shimmering, near-transparent fish that part around you like a living curtain and close again behind. These are glassfish and sweepers, and their massed, glittering clouds are among the most magical sights in the underwater world. Here's the story behind them.

The short answer: glassfish and sweepers are small, shimmering, often translucent fish that gather in huge shoals inside Red Sea caves, overhangs, and wrecks, creating dazzling living clouds. They're harmless, mesmerising, and a highlight of many dives.

What glassfish and sweepers are

Glassfish (also called glassy sweepers) are small fish with semi-transparent, glass-like bodies that catch and scatter light, giving them a shimmering, almost ghostly appearance. Sweepers are related small shoaling fish, often coppery or silvery, that similarly gather in dense numbers. Both species form enormous, tightly packed shoals in sheltered, shaded spaces. Individually tiny and unremarkable, en masse they become one of the reef's great spectacles — a shifting, glittering cloud that moves as one.

Where you'll find them

Glassfish and sweepers love shade and shelter, so they congregate in:

• Caves and caverns along reefs.
• Overhangs and crevices in the reef structure.
• Wrecks — the holds and interior spaces of ships like the famous Red Sea wrecks often fill with shimmering clouds of them, adding atmosphere to wreck dives.
• Coral heads and ergs with sheltered nooks.

When you enter one of these spaces and find it packed with shimmering fish, you've found them — and it's a moment most divers remember.

Why they gather like this

These fish shoal in massive numbers for protection. By clustering together in dense clouds in sheltered spaces, they reduce each individual's risk from predators — there's safety in numbers, and the shifting mass confuses hunters. The shade of caves and wrecks offers extra protection and a place to school by day. The downside (for them) is that predators like lionfish, groupers, and trevally know this too, and often patrol these spaces to hunt the shoals — so where you find glassfish, you frequently find their hunters lurking nearby, adding drama to the scene.

Why they're so beautiful to watch

The magic is in the light and movement. The fishes' translucent or reflective bodies catch the sunbeams filtering into caves and wrecks, making the whole shoal shimmer and sparkle. As you approach, the cloud parts around you and reflows, an immersive, almost dreamlike experience. Light beams, shadowed spaces, and a curtain of glittering fish make for some of diving's most atmospheric and photogenic moments — a favourite of underwater photographers.

How to enjoy them well

• Move slowly and gently into caves and overhangs so you don't scatter the shoal or stir silt.
• Mind your buoyancy in enclosed spaces — caves and wrecks are tight and fragile.
• Stay within your training for any overhead environments; cave and wreck penetration needs proper skills.
• Watch for predators hunting the shoals, and observe the natural drama without interfering.
• Bring a torch to reveal colours in shaded spaces, and a camera for the light beams.

Practical tips

Look for glassfish and sweepers in caves, overhangs, and wrecks on reef and wreck dives. Approach calmly to enjoy the parting-curtain effect. Keep good buoyancy and respect overhead-environment limits. Bring a torch for the shaded spaces and a camera for the shimmering light. And simply pause to take it in — these living clouds are one of the reef's quietest wonders.

Glassfish and sweepers prove that the reef's smallest residents can create its grandest spectacles. Swim into a shaded cave or wreck filled with their shimmering, shifting clouds, and you'll experience one of the Red Sea's most magical and atmospheric sights.

Want to swim through shimmering clouds of glassfish? Find cave and wreck dives on packnplan, and experience one of the Red Sea's most magical underwater spectacles.