For aquarium treatment, use 2–3 ppm methylene blue in water—about 1 teaspoon of 2.303% solution per 10 gallons (≈38 liters).
Methylene blue is a dye and medication that helps with egg fungus, protozoans, and nitrite stress in aquariums. The right amount in water depends on the goal: a full-tank course, a short bath, or an egg-hatching setup. Go too light and you get little benefit; go too strong and you can stress fish and upset your biofilter. This guide gives exact ranges, simple conversions, and clear mixing steps so you can dose with confidence.
How Much Dye To Add To Water Safely
The most used range for fish treatment is 2–3 ppm (mg/L). Many branded bottles are 2.303% strength, and the common label direction is 1 teaspoon (5 mL) per 10 gallons, which yields ~3 ppm. That single rule of thumb covers full-tank treatment and egg care across many setups. If you keep things metric, 10 gallons is about 37.9 liters, so 5 mL into ~38 L also lands near 3 mg/L. You can scale that up or down for any tank size.
What 2–3 Ppm Looks Like In Real Tanks
- Egg fungus prevention: Aim for ~3 ppm in a separate container with gentle aeration.
- Full-tank course: 2–3 ppm for 3–5 days in a hospital or quarantine tank, with daily checks.
- Short bath: 2–3 ppm for 30–60 minutes in a clean bucket or tub with an airstone.
Quick Reference Dose Chart (Early Look)
This table converts the common “1 teaspoon per 10 gal” rule into practical mixes you can apply today. It keeps things simple: the middle column covers full-tank use; the right column covers short baths at the same level.
| Tank/Container Size | Add 2.303% Solution For ~3 ppm | Add 2.303% Solution For ~2 ppm |
|---|---|---|
| 5 gal (19 L) | ½ tsp (2.5 mL) | ⅓ tsp (~1.7 mL) |
| 10 gal (38 L) | 1 tsp (5 mL) | ⅔ tsp (~3.3 mL) |
| 20 gal (76 L) | 2 tsp (10 mL) | 1⅓ tsp (~6.7 mL) |
| 40 gal (151 L) | 4 tsp (20 mL) | 2⅔ tsp (~13.3 mL) |
| 1 gal (3.8 L) bath | 10 drops (~0.5 mL) | 7 drops (~0.33 mL) |
| 2 gal (7.6 L) bath | 20 drops (~1 mL) | 14 drops (~0.67 mL) |
Notes: 1 teaspoon = 5 mL; 1 mL ≈ 20 drops with many droppers. Drop size varies by tip, so use a syringe or pipette for better accuracy.
When To Pick A Full-Tank Course Vs. A Short Bath
Full-Tank Course (2–3 Ppm For 3–5 Days)
Use a full-tank course for widespread symptoms in a hospital or quarantine tank. Keep aeration strong and pull carbon from filters since it removes dye. A partial water change at the end helps clear the water before you add carbon back.
Short Bath (2–3 Ppm For 30–60 Minutes)
Use a short bath for quick hits on early signs or after shipping stress. Place the fish in a separate, well-aerated container. Watch behavior the whole time. If the fish gasps or shows clear distress, move it back to clean, matched-temperature water.
Egg Care Setup (~3 Ppm)
Place eggs in a small container with the dye mix and an airstone. Gentle flow keeps fungus from jumping to healthy eggs. Keep the container bare so you can spot bad eggs early and remove them.
Safe Mixing, Step By Step
- Measure the water volume. Use tank inside dimensions or a volume calculator. For tubs and buckets, mark fill lines so you can repeat the dose later.
- Pull carbon media. Carbon strips dye fast. Keep mechanical sponges running for circulation.
- Pre-mix in a cup. Add the measured dye to a cup of tank water, swirl, then pour around the tank for even color.
- Add air. A small airstone helps. The dye tints water and can lower light penetration a bit; strong aeration offsets that.
- Log what you did. Write down date, target ppm, and volume added. Your future self will thank you.
How The Math Works (So You Can Scale Any Dose)
Most aquarium bottles are 2.303%, which means 2.303 g dye per 100 mL solution. The common direction—5 mL per 10 gal—gives about 3 mg/L in ~38 L. That becomes your base unit. From there, each extra mL per 10 gal adds about ~0.6 mg/L (since 5 mL → ~3 mg/L). In practice, stick to the 2–3 mg/L band unless a label for your exact product states a different target.
Metric Shortcut
For 2–3 mg/L, the dose per liter of tank water is small. Using the standard bottle, you’ll add around 0.13 mL/L for 3 mg/L (because 5 mL divided by 37.9 L ≈ 0.132). For 2 mg/L, add about 0.09 mL/L. A 1 mL syringe makes these micro additions easy and repeatable.
Filtration, Plants, And Biofilter Care
Dye treatment can stress or suppress nitrifying microbes. Keep ammonia and nitrite at zero with daily tests during and after dosing. If you see a bump, do a partial change and add seeded media from a healthy tank. Sponge filters tolerate dye courses well. If you grow sensitive plants, move fish to a plain hospital tub, then return them after the course so you don’t stain leaves or knock back growth.
Manufacturer directions on many aquarium bottles match the ranges above and also remind you to remove carbon during treatment. You can read a typical label’s math and timings on the Kordon instruction page linked below.
Also note the warning many experienced keepers share: strong dye levels can weaken the biofilter. There’s published work showing that methylene blue can harm nitrifying microbes, which explains why you may see a mini-cycle after a heavy hand. Keep tests handy and feed lightly during a course.
Kordon methylene blue directions describe the common “1 tsp per 10 gal” → ~3 ppm rule, and a peer-reviewed study on nitrifier sensitivity explains why careful dosing and monitoring help stability.
Troubleshooting Dosing Mistakes
Water Is Way Too Dark
Do a 30–50% change and top back up with matched temperature. Add fresh aeration. If you pulled carbon earlier, add it back after the change to help strip the tint, then remove it again if you plan to re-dose.
Fish Are Gasping Near The Surface
Move the fish into clean, aerated water with the same temperature and salinity. Add an airstone to the main tank. Recheck your math before any second attempt.
Ammonia Or Nitrite Spikes
Change water, dose a proven bacterial starter, and keep feeding light. Run a small bag of carbon only after you finish the course, not during it.
Mixing Recipes You Can Trust (Stock Sizes And Targets)
Use this late-stage table as a ready reckoner when you’re mid-treatment. It lists how much 2.303% solution to add for common targets into small and medium tanks. Round to the nearest 0.5 mL when you don’t have a fine-scale syringe.
| Target Level | Add Per 10 gal (38 L) | Add Per 40 L |
|---|---|---|
| 1 mg/L (1 ppm) | ~1.7 mL (about ⅓ tsp) | ~1.8 mL |
| 2 mg/L (2 ppm) | ~3.3 mL (about ⅔ tsp) | ~3.5 mL |
| 3 mg/L (3 ppm) | 5 mL (1 tsp) | ~5.3 mL |
| Bath at 2 mg/L | ~3.3 mL per 10 gal of bath water | ~3.5 mL per 40 L of bath water |
| Egg care at ~3 mg/L | 5 mL per 10 gal of egg water | ~5.3 mL per 40 L of egg water |
Care Notes And Limitations
- Do not mix with carbon running. Carbon strips dye.
- Keep light feeding. Extra waste stacks the deck against stressed gills.
- Mind tankmates. Invertebrates and live plants may react badly. Use a bare hospital tub if unsure.
- Wear old clothes and gloves. The dye stains fingers, silicone, towels, and anything porous.
- Store the bottle safely. Keep it sealed, upright, away from kids and pets.
What About Other Strengths On The Label?
Some regions sell different concentrations. If your bottle isn’t 2.303%, follow the manufacturer’s printed rate for their exact strength. If the label gives ppm targets, you can still use the tables here—just match the ppm and adjust the mL using your label’s “mL per X gallons” rule. When in doubt, start at the low end of the range with strong aeration.
Simple Calculations For Any Brand
Step 1: Confirm bottle strength from the label. If it reads 2.303%, the 1 tsp per 10 gal rule lands near 3 ppm.
Step 2: Decide on 2 or 3 ppm based on the use case. Egg fungus and severe stress often warrant the upper end in a separate container. General courses for protozoans often use the same range in a hospital tank.
Step 3: Convert to your tank size. Divide the rule by two for 5 gal, double for 20 gal, and so on. Use syringes for clean mL dosing.
Step 4: Watch behavior, test ammonia and nitrite daily, and change water if fish seem off. Add carbon only after the course if you want clear water fast.
When Not To Dose The Display Tank
Skip the main display if it’s heavily planted or if you’ve built up a delicate biofilter. Move fish to a plain tub or hospital tank. That way, you protect plants and bacteria from staining and stress while still giving the fish the help they need.
Quick Answers To Common Mixes
- Ten-gallon tank to ~3 ppm: Add 5 mL of 2.303% solution.
- Twenty-gallon tank to ~2 ppm: Add ~6.7 mL.
- One-gallon bath to ~2 ppm: Add ~0.33 mL (about 7 drops).
- Egg tub at ~3 ppm, 2 gallons: Add ~1 mL (about 20 drops) with an airstone.
Last Checks Before You Dose
- Confirm you pulled carbon.
- Confirm your mL math and tank volume.
- Set a timer for the bath or a reminder for the day count on a course.
- Keep a test kit within reach for ammonia and nitrite.
Why This Range Works
The 2–3 ppm band is widely used because it hits pathogens hard while staying manageable for most fish when applied with aeration. It’s also easy to remember and scale. Labels from major aquarium brands point to the same figures, and long-time keepers rely on that match between ppm math and a simple teaspoon rule.
Wrap-Up: Dose Smart, Watch Closely
Set the target (2–3 ppm), remove carbon, add strong air, and track behavior. Use a hospital tub when plants or the display biofilter needs protection. Clear the water with changes and carbon once you finish the course. With clean math and steady checks, methylene blue can be a handy tool in your fish-care kit.