The standard dose is 2 to 5 drops per gallon of water, added once daily for three to five days

Methylene blue treats fungal and parasitic infections in fish, and the amount you use depends on your tank size and the severity of the infection. A 5-gallon tank gets 10 to 25 drops per dose. A 20-gallon tank gets 40 to 100 drops. A 55-gallon tank gets 110 to 275 drops. The standard concentration sold in pet stores is usually 1% methylene blue, which means each drop contains a small, consistent amount of the active ingredient.

Always measure your tank's actual volume before dosing—many tanks hold less than their advertised size because of gravel, decorations, and the space the filter takes up. If you are unsure, fill the tank with a measuring cup to find the true volume. Overdosing will not kill the fish faster, but it can stress them and stain your tank and decorations blue for weeks.

Key Takeaways

  • The standard dose is 2 to 5 drops per gallon, added once daily for three to five days in a row.
  • Measure your tank's actual volume before dosing, because advertised sizes often do not match the water you actually add.
  • Methylene blue stains everything it touches blue, so expect discoloration of gravel, plants, and silicone that will fade over time.
  • Stop dosing and do a 50% water change if fish show signs of stress, such as gasping at the surface or hiding constantly.
  • Some fish, such as plecos and corydoras catfish, are sensitive to methylene blue and may need a half dose or no treatment at all.

Why the dose depends on tank size and water volume

Methylene blue works by reaching a certain concentration in the water. A small tank needs fewer drops to reach that concentration than a large tank does. If you add the same number of drops to a 5-gallon tank and a 55-gallon tank, the small tank will be much more concentrated and may stress or harm the fish.

The 2 to 5 drops per gallon range gives you room to adjust based on what you see. Start at the lower end—2 drops per gallon—if the infection is mild or if you are treating for the first time. Use the higher end—5 drops per gallon—if the infection is severe or if the fish do not improve after two days at the lower dose. Never guess at the volume. If your tank is 18 gallons, use 36 to 90 drops, not a handful.

How to measure and add methylene blue safely

Use a dropper or syringe to measure the dose. Most methylene blue bottles come with a dropper cap, but if yours does not, ask the pet store for a dropper or buy one separately—guessing at drops will lead to overdosing. Draw the correct number of drops into the dropper, then add them to the tank near the filter intake so the water current spreads the medicine evenly.

Add the dose once per day at the same time each day, usually in the morning. Treat for three to five days in a row, then stop. Do not continue dosing after five days unless a veterinarian tells you to. After the treatment period ends, do a 25% water change to begin clearing the blue color from the tank. The staining will fade on its own over the next week or two as you do regular water changes.

Fish that should not be treated with methylene blue

Some fish are sensitive to methylene blue and may be harmed by the standard dose. Plecos, corydoras catfish, and other bottom feeders have sensitive skin and should receive half the normal dose or be treated in a separate hospital tank if possible. Labyrinth fish such as bettas and gouramis are also sensitive, though they tolerate half doses better than they tolerate full doses.

If you have sensitive fish in your tank, consider moving the sick fish to a hospital tank—a small separate aquarium where you can treat it without exposing the sensitive fish to methylene blue. A 5 or 10-gallon hospital tank is inexpensive to set up and gives you more control over the treatment. If you cannot move the fish, use half the normal dose and watch closely for signs of stress.

Signs that the dose is too high

Fish that are stressed by methylene blue will gasp at the surface, hide constantly, or stop eating. If you see any of these signs within an hour of dosing, do a 50% water change when ready to dilute the medicine. This will not harm the fish and will usually bring them back to normal behavior within a few hours.

Methylene blue can also reduce oxygen in the water if the dose is very high, so make sure your filter and air stone are running at full strength during treatment. If your tank does not have an air stone, consider adding one temporarily while you are treating with methylene blue. The extra oxygen will help the fish stay comfortable and will also help the medicine work more effectively.

What methylene blue treats and when to use it

Methylene blue is most effective against fungal infections (white fuzz on the body or fins) and parasitic infections such as ich (white spots that look like salt grains). It is less effective against bacterial infections, which usually need antibiotics instead. If the fish has torn fins or open sores, methylene blue can help prevent secondary fungal infection while the wound heals.

Methylene blue is safe to use alongside other treatments in most cases, but do not mix it with salt, antibiotics, or other medications without checking the label or asking a veterinarian first. Some combinations can reduce the effectiveness of both treatments or harm the fish. If you are unsure whether methylene blue is the right treatment for your fish's problem, describe the symptoms to a veterinarian or experienced aquarium keeper before dosing.

Cleaning up methylene blue stains after treatment

Methylene blue will stain your gravel, decorations, and silicone sealant blue. The stain is harmless and will fade as you do water changes, but it can take two to four weeks to disappear completely. You can speed up the process by doing 25% water changes every other day instead of once a week, but this is not necessary unless the blue color bothers you.

If you want to remove the stain faster, activated charcoal in your filter will absorb some of the methylene blue and clear the water more quickly. Add a small bag of activated charcoal to your filter after the treatment period ends, and replace it after a week. The stain on gravel and decorations will still fade slowly, but the water will clear in a few days instead of a few weeks.

Frequently Asked Questions

Can I use methylene blue if I have live plants in the tank?

Methylene blue will not kill most aquarium plants, but it will stain them blue and may reduce the light they receive while the water is colored. If you have delicate plants such as mosses or stem plants, consider moving them to a separate container during treatment, or use a hospital tank instead. Hardy plants such as anubias and java fern will survive the treatment without problems.

How long does methylene blue last in the water?

Methylene blue breaks down and fades over time, especially in tanks with strong lighting or active filters. The water will be noticeably less blue after three to five days, and the color will continue to fade for two to four weeks. You do not need to do water changes to remove it—regular maintenance water changes will clear it gradually.

What if I accidentally overdose methylene blue?

Do a 50% water change when ready to dilute the medicine. Watch the fish closely for the next few hours for signs of stress such as gasping or hiding. If the fish seem fine after a few hours, they will likely recover without problems. Overdosing is uncomfortable for the fish but rarely fatal if you catch it and do a water change quickly.

Can I use methylene blue in a tank with invertebrates like shrimp or snails?

Methylene blue is toxic to most invertebrates, including shrimp, crabs, and some snail species. If your tank has shrimp or other invertebrates, move them to a separate container before treating, or use a hospital tank for the sick fish instead. This is the safest way to treat the fish without harming the invertebrates.

Do I need to turn off the light while treating with methylene blue?

No, you do not need to turn off the light. Methylene blue works in light or darkness. However, strong lighting may speed up the breakdown of the medicine, so if you have very bright lighting, you might see the blue color fade faster than expected. This does not mean the treatment is not working—it just means you may need to redose on schedule even if the water is not very blue.