Ammonia or nitrite
New tanks, overfeeding, a dead fish, or an over-cleaned filter can cause spikes that damage fins and gills.
Sick fish guide
Fin Rot and Torn Fins
Ragged fins usually come from an injury, like nipping or snags, or from fin rot, a bacterial infection that takes hold when water quality slips or fish are stressed. Test the water first. Clean, stable water heals most fins without medication.
Symptoms
Fin rot or a torn fin?
The fin edge tells you a lot. Take a photo so you can compare tomorrow.
- Frayed, uneven edges that get shorter over several days, often on more than one fin.
- A discolored margin: a milky white, gray, red, or dark line along the damaged edge.
- Red streaks or bloody spots in the fin or at its base mean the infection is active.
- Clamped fins and low energy: fins held tight, hanging near the bottom, eating less.
- Rot reaching the body: this is serious. Fins may not grow back if the base is damaged.
A torn fin from an injury usually has a clean split or bite-shaped notch with a normal-colored edge that is not spreading. Most heal on their own in clean water.
- Only long-finned fish damaged? Bettas, fancy guppies, and angelfish are targets for nippy tankmates like tiger barbs or serpae tetras.
- Cotton-like tufts, or a white patch spreading on the body or mouth? That can be a faster infection such as columnaris or fungus. Call or bring a photo right away.
- Healing fins grow back with a thin, clear or whitish edge. That is new tissue, not rot.
Saltwater fish get ragged fins too, most often from aggression or poor water. Treat them in a separate tank, since many medications harm corals and invertebrates.
What to test first
Check the water before the fish.
Fin rot almost always follows a problem in the tank. Find it, or fins keep breaking down.
Check
What you are looking for
If it is off
Ammonia and nitrite Liquid test kit.
Both should read zero in an established tank. Any reading is a likely trigger.
Do a partial water change today, then find the source: overfeeding, a dead fish, or an over-cleaned filter.
Nitrate Same day.
Over about 40 ppm in freshwater means water changes are overdue.
Do smaller, more frequent water changes until it comes down.
Temperature Separate thermometer.
A failing, undersized, or mis-set heater lets temperature swing.
Most tropical community fish want a steady 75 to 80°F. Fix or replace the heater.
pH and KH Especially older tanks.
Low KH lets pH drop or swing, which stresses fish and slows healing.
Raise KH slowly with regular water changes. See the pH swings guide at the bottom of this page.
Tankmates and decor Watch at feeding time.
Chasing, nipping, rough rock, stiff plastic plants, or an uncovered intake.
Separate the nipper or the injured fish. If thin nylon tights snag on decor, so can fins.
Likely causes
What usually starts it?
Fin rot bacteria live in almost every tank. Stress and dirty water let them win.
Overdue water changes
Nitrate and waste build up slowly. Fish look fine for weeks, then fins start to fray.
Fin nippers and bullies
Some barbs, tetras, and territorial fish target long fins. In saltwater, aggression is the top cause of torn fins.
Sharp decor and equipment
Rough rock, stiff plastic plants, nets, and open intakes tear fins, and torn edges can get infected.
Temperature swings and crowding
Shipping, crowding, and unstable temperature weaken fish so small injuries do not heal.
A disease came first
Parasites or wounds can leave fins open to a secondary infection. Look for other symptoms too.
What to do today
Clean water first, medicine last.
Most mild cases improve within a week or two once the water is right.
1
Test
Test ammonia, nitrite, nitrate, pH, and temperature.
Write the numbers down. If ammonia or nitrite is above zero, see the ammonia or nitrite guide.
2
Water change
Do a partial water change.
Change about 25% with dechlorinated, temperature-matched water. Repeat over the next few days if ammonia or nitrite is up.
3
Clean
Remove waste gently.
Vacuum part of the gravel, remove leftover food, and rinse the filter sponge in old tank water, not tap water.
4
Remove the cause
Separate nippers and fix sharp decor.
Move the bully or the injured fish, swap rough decor for smooth pieces, and cover intakes with a sponge.
5
Watch
Feed lightly and check daily.
Compare photos each day. A clear edge of new growth means healing. Full regrowth can take weeks.
6
Treat if needed
Medicate only if it keeps spreading.
If rot advances after several days of clean water, use an antibacterial treatment labeled for fin rot, in a hospital tank when possible. Remove carbon, follow the label exactly, and keep aeration high.
What to avoid
Mistakes that slow healing.
Rushed treatment does more harm than a few days of clean water.
- Medicating before fixing the water. No treatment keeps up if the water keeps causing damage.
- Antibacterial treatments "just in case." Save them for rot that is clearly spreading. Overuse harms filter bacteria and helps resistant bacteria survive.
- Treating the main tank when you can avoid it. A hospital tank protects plants, shrimp, snails, and your filter. See quarantine and hospital tanks.
- Mixing medications or starting a second one before the first course is done.
- Leaving carbon in while medicating. Carbon pulls medication out of the water.
- One giant water change in a neglected tank. It can shift pH and hardness too fast. Do several smaller changes over a week.
- Adding aquarium salt without checking. It is not safe for every fish, live plants, shrimp, or snails. Ask first.
Helpful supplies
Store departments that help.
Start with testing and water changes. Treatment comes second.
Liquid test kits
Ammonia, nitrite, nitrate, and pH show whether water is behind the damage.
View testingSiphons and gravel vacs
Easy, regular water changes treat mild fin rot and help prevent it.
Shop maintenanceConditioners and fin rot treatments
Conditioner for every water change. Pick a freshwater or saltwater antibacterial only after the water is fixed.
View additivesA small bare treatment tank
A lid, heater, and sponge filter make treatment safer for the fish and the display.
View aquariumsAir pumps and sponge filters
Gentle filtration for a hospital tank and extra oxygen while medicating.
View air equipmentHeaters and thermometers
A correctly sized heater keeps temperature steady so fish can heal.
View heatingWhen to bring a water sample or call
Call or come in if rot reaches the body, you see red streaks or cotton-like patches, several fish are affected, or a fish stops eating. Bring a fresh water sample with your tank size, livestock list, and recent changes, plus a clear photo or short video.
Call (215) 269-4930 Visit the store