“What is this in my tank?”
You spotted something — tiny worms on the glass, a thread waving from the substrate, a cloudy bloom, a strange jelly-like blob. Before you reach for chemicals, find out what it actually is. Most of what shows up in an aquarium is harmless or even beneficial, and the few real pests are best handled by fixing their cause. Each entry tells you exactly what you are looking at, whether to worry, and what to do.
The one rule before you treat anything
Almost every “infestation” in an aquarium is a symptom, not the disease. A sudden bloom of worms, snails, or biofilm is the tank telling you there is more food than the system can process — usually from overfeeding or accumulated detritus. Nuke the population with chemicals and it comes back, because the cause is still there. Cut the food supply and improve maintenance, and most populations shrink on their own.
So the workflow for any mystery organism is the same: identify it, decide whether it is actually a threat (most are not), address the underlying cause, and only then consider direct removal. The entries below follow that order. Many of these organisms are also covered in depth — including the beneficial ones you might want to culture on purpose — in the Microfauna Database.
Identify it
Detritus Worms
HarmlessHarmless, and even beneficial in small numbers. A visible bloom is a signal of overfeeding or a dirty substrate, not an infection.
Green Water in Aquarium — What Causes It and How to Clear It
HarmlessHarmless to fish and even beneficial as fry food, but unsightly. Caused by excess light and nutrients.
Hydra
Moderate riskHarmless to adult fish, but a real predator of fry and baby shrimp. A bloom almost always means overfeeding.
Planaria
Moderate riskHarmless to fish, but predatory toward shrimp eggs, baby shrimp, and weak shrimp. Almost always caused by overfeeding.
Tiny White Worms in Your Fish Tank — What They Are and What to Do
Low riskAlmost always harmless detritus worms or nematodes; occasionally planaria, which is a shrimp risk. Identify before acting.
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