Overview
Hydroids (class Hydrozoa) are one of the most common “tiny pests you don’t notice until they’re everywhere.” They usually show up as small, fuzzy, hair-like, or starburst-looking polyps on rock, frag plugs, overflow walls, and sometimes even on snail shells or frag racks. Some stay small and harmless-looking. Others spread into annoying patches that sting corals, irritate polyps, and make you dread looking at your frag plug bases.
The frustrating part is that “hydroids” isn’t one single creature in the way “Aiptasia” is. It’s a group of related organisms that show up in tanks in a similar pesty way. For the hobby, the practical point is: if you’ve got little stinging colonial polyps multiplying where you don’t want them, you’re probably dealing with pest hydroids.
They’re not a tank-ending apocalypse most of the time — but they’re perfectly capable of becoming a chronic irritation if you let them establish.
Quick Care Snapshot
Common Name: Hydroids
Scientific Name: Hydrozoa (pest hydroids; multiple genera/species)
Reef Safe: No (many sting corals and irritate tissue)
Difficulty: Easy to “keep,” annoying to eliminate
Temperament: Stinging colonial polyps
Lighting: Tolerant (depends on type; many don’t care)
Flow: Often favor moderate flow areas where food passes by
Diet: Micro-foods, suspended particles, tank “soup”
How They Spread: Budding/colonies, fragmentation, settlement of new polyps
Main Risk: Coral irritation, spreading on plugs/racks, chronic nuisance
Natural Background
Hydrozoans are cnidarians, related in a broad sense to corals, anemones, and jellyfish. Many have a life cycle that includes tiny polyps attached to surfaces. In the ocean, they grow on hard substrate and feed on microplankton.
In aquariums, they thrive because we provide:
• Constant food particles
• Stable salinity and temperature
• Tons of hard surfaces (plugs, racks, glass, rocks)
• Little predation pressure for microscopic/colonial pests
Tank Requirements
If your tank has:
• hard surfaces
• food in the water column
• stable reef conditions
…hydroids can survive.
They tend to show up most in:
• Frag systems
• Newer tanks with “hitchhiker explosions”
• Systems that feed coral foods heavily
• Areas with moderate flow that carries food past the colony
Where to look
• Frag plug bases (especially around glue lines)
• Frag racks
• Overflow boxes
• Sump walls and baffles
• The shaded sides of rock structures
Feeding
Hydroids feed the same way many small polyps do:
• Tiny suspended foods
• Fine particulate coral foods
• Microplankton-size particles
• General “reef soup” (organics and micro stuff)
This is why they often bloom in tanks where:
• Coral foods are used heavily
• Feeding is frequent
• Detritus is allowed to stay in suspension
You don’t have to “feed” hydroids for them to grow — you feed your reef, and they take advantage.
Compatibility
With corals
This is where they become a problem:
• Some hydroids sting nearby coral tissue
• They can cause persistent polyp retraction
• They can spread across surfaces close to coral bases
• They can irritate soft corals and zoas in particular when they settle near mats
With fish and inverts
Fish usually ignore them. Many cleanup crew members can’t (or won’t) eat them because they’re tiny, stinging, and often protected within a colony structure.
Hydroids are often a “nuisance layer” pest — not big enough to get targeted by predators, but annoying enough to mess with your coral comfort zone.
Common Mistakes
1) Assuming they’re “just harmless fuzz”
Some are relatively harmless, but many do sting. If corals near them are staying retracted, assume they’re not friendly.
2) Scrubbing without containment
If you scrub colonies off racks or plugs inside the tank, you can spread fragments and encourage settlement elsewhere.
3) Ignoring the plug/rack problem
Hydroids love frag plugs and rack surfaces. If you don’t manage them there, they become a constant source of reintroduction.
4) Overreacting with tank-wide chaos
Hydroids are annoying, but swinging parameters or nuking stability often harms your reef more than the hydroids do.
5) Not differentiating hydroids from other hitchhikers
Hydroids can be confused with:
• bryozoans
• small feather dusters
• spirorbid worms
• harmless colonial filter feeders
The difference is often behavior and irritation: if it’s a tiny polyp colony that seems to sting or cause local coral retraction, hydroids jump to the top of the suspect list.
Notes & Variations
“Hydroids” is a catch-all in the hobby
There are many types that can show up, including:
• hair-like colonies
• starburst polyps
• tiny branching “weed” growths
• small anemone-like polyps
Instead of trying to perfectly taxonomize them, focus on:
• where they grow
• whether they sting/irritate
• how fast they spread
• how they respond to removal
Why they often show up in frag systems
Frag setups often have:
• high feeding for growth
• lots of plugs/racks
• high surface area
• lots of new coral imports
That’s hydroid paradise.
Final Thoughts
Hydroids are one of those pests that are less about fear and more about discipline. They reward neglect, especially on frag plugs and racks. But they also respond well to consistent management and early intervention.
If your goal is a clean, comfortable coral environment, hydroids are worth addressing early — not because they’ll destroy everything overnight, but because they’re excellent at becoming a permanent background annoyance if you let them.