What do I do about white spots on a fish?

Freshwater white spot is Ichthyophthirius multifiliis: isolate the affected fish, raise the temperature to 30 to 32 °C to speed up its life cycle, and add salt to 0.3 to 0.5 percent, holding that for 10 to 14 days and for a further week after the last spot disappears. Scaleless fish such as loaches, plecos and Corydoras tolerate neither salt nor copper, so for them the treatment is heat plus water changes. Marine white spot is a different parasite, Cryptocaryon irritans, and needs copper or hyposalinity, with the display left fallow for at least 76 days before fish go back in.

What temperature should ich treatment actually reach?

The freshwater ich protocol uses 30 to 32 °C, but only if the fish tolerates it. Cold water species such as goldfish, koi and white cloud mountain minnows cannot be taken to that temperature and have to be treated with salt and water changes instead. The purpose of the heat is to shorten the parasite life cycle so the trophonts leave the fish sooner, which means it only works alongside gravel vacuuming and water changes that remove the cysts. Raising the temperature without siphoning the substrate leaves the parasite in the tank.

Why does fungus keep coming back?

Because a fungus infection is almost always secondary and only grows on tissue that is already damaged. Repeated outbreaks mean the primary cause is still present, most often an injury from fighting, a sudden drop in temperature, or ammonia and nitrite sitting high for weeks. Killing the fungus with an antifungal leaves the wound open and the water unchanged, so it returns within days.

Is fin rot bacterial or a water quality problem?

Usually it starts with water quality and becomes bacterial afterwards. A pale, ragged, shortened fin edge most often begins with ammonia or nitrite irritation on top of an injury from fighting, and bacteria such as Flavobacterium columnare or Vibrio then colonise the wound. Treat it in that order: water changes until ammonia and nitrite read zero, an antibacterial only if the erosion continues, and a change to the stocking that caused the fighting.

Will medication kill the nitrifying bacteria and the corals?

Most antibiotics and antiparasitics damage the nitrifying bacteria to some degree, so cut feeding in half while dosing, test ammonia and nitrite daily and keep water ready for a change. For corals, shrimp, snails and the life on live rock, copper and formalin are lethal. Every disease entry in the atlas that uses them is flagged as unsafe for inverts, and those treatments have to run in a quarantine tank while the display stays fallow for the stated period.

G-04 · Freshwater / Marine

Disease atlas

Every disease entry in the atlas follows the same structure: causes, symptoms, a step-by-step treatment plan, and the medications used with their contraindications. The order is deliberate, because three things go wrong otherwise. Symptoms such as flashing, clamped fins and dark colouring also appear when water quality degrades, so medicating immediately hides the real cause. Most medications also damage the nitrifying bacteria, so ammonia rises again after dosing. And the effective drugs in this hobby, copper and formalin above all, are lethal to corals, shrimp, snails and everything living on the live rock, so one dose in the wrong tank ruins it.

Isolate first, test the water second and medicate third, because most cases of a sick fish arrive with new stock or follow a water quality crash.

Units: temperature in °C, hardness in German degrees (dGH / dKH), marine salinity as specific gravity at 25 °C, volume in litres. These are the values recorded for the species and do not follow the site unit switcher.

Cause

Severity

Contagion

13 of 13 entries

Disease atlas
Name	Cause	Severity	Contagion	Corals / inverts
Freshwater Ich白点病	Parasite	Severe	Highly contagious	Not safe
Freshwater Velvet丝绒病	Parasite	Severe	Highly contagious	Not safe
Gill Flukes鳃吸虫	Parasite	Moderate	Highly contagious	Not safe
Internal Parasites体内寄生虫	Parasite	Moderate	Low contagion	Not safe
Saprolegniasis水霉病	Fungus	Moderate	Low contagion	Not safe
Fin and Tail Rot烂鳍烂尾病	Bacteria	Moderate	Highly contagious	Not safe
Columnaris棉口病	Bacteria	Severe	Highly contagious	Not safe
Dropsy腹水病	Bacteria	Severe	Low contagion	Not safe
Popeye凸眼病	Bacteria	Moderate	Low contagion	Not safe
Hole in the Head头洞病	Nutrition	Moderate	Not contagious	Safe
Lymphocystis淋巴囊肿	Virus	Mild	Low contagion	Safe
Ammonia and Nitrite Poisoning氨氮与亚硝酸盐中毒	Water quality	Severe	Not contagious	Safe
pH Shock酸碱震荡	Water quality	Severe	Not contagious	Safe

How to use it

Step 1Match the symptom firstIch looks like grains of salt, velvet like a dusting of flour, fungus like cotton wool, and fin rot like a pale eroded fin edge. Different shapes mean different causes.
Step 2Confirm the water type and tank typeFreshwater and marine pathogens are different organisms, and a shared name such as white spot covers two unrelated parasites with treatments and doses that do not transfer.
Step 3Test the water before you decide to medicateMeasure ammonia, nitrite, pH and temperature first. Only a tank with sound water quality moves on to the treatment steps in order.
Step 4Read the invertebrate safety noteAnything marked as unsafe for corals, shrimp and snails has to be dosed in a quarantine tank, and the display has to stay fallow for the period given in the entry.

Frequently asked questions

What do I do about white spots on a fish?: Freshwater white spot is Ichthyophthirius multifiliis: isolate the affected fish, raise the temperature to 30 to 32 °C to speed up its life cycle, and add salt to 0.3 to 0.5 percent, holding that for 10 to 14 days and for a further week after the last spot disappears. Scaleless fish such as loaches, plecos and Corydoras tolerate neither salt nor copper, so for them the treatment is heat plus water changes. Marine white spot is a different parasite, Cryptocaryon irritans, and needs copper or hyposalinity, with the display left fallow for at least 76 days before fish go back in.
What temperature should ich treatment actually reach?: The freshwater ich protocol uses 30 to 32 °C, but only if the fish tolerates it. Cold water species such as goldfish, koi and white cloud mountain minnows cannot be taken to that temperature and have to be treated with salt and water changes instead. The purpose of the heat is to shorten the parasite life cycle so the trophonts leave the fish sooner, which means it only works alongside gravel vacuuming and water changes that remove the cysts. Raising the temperature without siphoning the substrate leaves the parasite in the tank.
Why does fungus keep coming back?: Because a fungus infection is almost always secondary and only grows on tissue that is already damaged. Repeated outbreaks mean the primary cause is still present, most often an injury from fighting, a sudden drop in temperature, or ammonia and nitrite sitting high for weeks. Killing the fungus with an antifungal leaves the wound open and the water unchanged, so it returns within days.
Is fin rot bacterial or a water quality problem?: Usually it starts with water quality and becomes bacterial afterwards. A pale, ragged, shortened fin edge most often begins with ammonia or nitrite irritation on top of an injury from fighting, and bacteria such as Flavobacterium columnare or Vibrio then colonise the wound. Treat it in that order: water changes until ammonia and nitrite read zero, an antibacterial only if the erosion continues, and a change to the stocking that caused the fighting.
Will medication kill the nitrifying bacteria and the corals?: Most antibiotics and antiparasitics damage the nitrifying bacteria to some degree, so cut feeding in half while dosing, test ammonia and nitrite daily and keep water ready for a change. For corals, shrimp, snails and the life on live rock, copper and formalin are lethal. Every disease entry in the atlas that uses them is flagged as unsafe for inverts, and those treatments have to run in a quarantine tank while the display stays fallow for the stated period.