Brooklynella hostilis is a parasite found in marine aquaria, which causes the clownfish disease. Hosts of this parasite include jawfishes, surgeonfishes, seahorses, wrasses, tangs and angelfishes.

When this parasite causes clownfish, fishes can be treated by bathing them in fresh water, but with the same pH levels of the normal tank. Another treatment suggestion includes a solution with malachite green and copper combined with formaldehyde can help reduce its symptoms. However, this type of treatment is not recommended as it is mostly ineffective. The best available treatment is formaldehyde alone.

Symptoms of brooklynella hostilis usually start by simple fish actions. The host may scrap against objects to remove the parasite, some experience rapid respiration and difficulty of breathing. Fishes may gasp for air since the gills become clogged with mucus. Fishes refuses to eat and becomes lethargic. Their colors fade and heavy amounts of slime is produced. This slimy mucus starts to cover the head and spreads outward with skin lesions appearing. Some hosts develop signs of secondary bacterial infections.