Seachem Prime Water Conditioner: Preparing Safe Water for Your Tank

Water quality · How-to guide · About 5 minutes · Beginner friendly

The key habit: Use Prime every single time you add new water to your aquarium—at water changes, during initial setup, and even when topping off. It removes chlorine and chloramine from tap water and detoxifies ammonia, nitrite, and nitrate.

Shop the Seachem Prime 100ml

In this guide

1. What Prime does and when you need it
2. Choosing the right size
3. How to dose Prime correctly
4. Adding Prime to your water
5. Special situations and dosing adjustments
6. Safety and storage

1. What Prime does and when you need it

Seachem Prime is a complete and concentrated conditioner for both fresh and saltwater aquariums. Most tap water contains chlorine or chloramine added by municipalities to kill bacteria—but these chemicals are harmful to fish, corals, and the beneficial bacteria in your filter. Prime removes chlorine and chloramine, and detoxifies ammonia, nitrite, and nitrate through a binder which renders these non-toxic, allowing the biofilter to more efficiently remove them.

Use Prime at start-up and whenever adding or replacing water. It's also valuable during the cycling stage of a new tank. Prime may be used during tank cycling to alleviate ammonia/nitrite toxicity. When used during cycling, Prime enhances nitrate removal and does not cause a pH drop or overactivate protein skimmers.

2. Choosing the right size

Prime comes in three sizes. The shop carries the 100 mL bottle, which treats 2,000 gallons of water. A single capful (5 mL) treats 200 gallons, so a 100 mL bottle provides roughly 20 capfuls. For a 50-gallon tank doing a 25-gallon monthly water change, one bottle lasts about four months of regular maintenance. For smaller tanks or frequent water changes, a smaller size makes sense; for large systems or multiple tanks, larger bottles are economical.

3. How to dose Prime correctly

Use 1 capful (5 mL) for each 200 L (50 US gallons) of new water. For smaller volumes, each cap thread is approximately 1 mL. This means for a 10-gallon water change, you would use about 1 mL; for 25 gallons, about 1.25 mL; for 50 gallons, one full capful.

The most important thing: measure Prime before you mix or add it. If your tap water adds 10 gallons to your tank, dose based on 10 gallons—not the size of the tank. If you're adding water directly to the aquarium, then base your dose on the total tank volume.

A standard dose removes approximately 1 mg/L ammonia, 4 mg/L chloramine, or 5 mg/L chlorine.

4. Adding Prime to your water

Gather before you start

• Clean container (bucket) for new water
• Seachem Prime bottle with measuring cap
• Measuring spoon or scale (optional, for very small doses)
• Tap water
• Thermometer (for large water changes)

1. Fill your water container with fresh tap water. Let it sit a moment so it reaches room temperature or the temperature of your aquarium.

2. Measure the right amount of Prime based on the water volume you're adding. If you're unsure, err on the side of using a tiny bit too much rather than too little—Prime is very safe.

3. Add Prime to the new water. Prime may be added to aquarium directly, but better if added to new water first. This is the preferred method: mix Prime into your bucket of new water, wait a moment, then pour into the tank. This ensures even distribution.

4. Wait a few seconds, then add your conditioned water to the aquarium.

A sulfur odor is normal. This is not a problem and does not affect your tank.

5. Special situations and dosing adjustments

High chloramine: For exceptionally high chloramine concentrations, a double dose may be used safely. If your area uses very high levels of chloramine in the water supply, double your normal dose.

Emergency ammonia spike: To detoxify nitrite in an emergency, up to 5 times normal dose may be used. If you test your water and find dangerous ammonia or nitrite levels (above 2 ppm), you can use 5 times the normal dose as a temporary measure while you work to address the root cause (usually overfeeding or overcrowding).

Very warm water: If temperature is > 30 °C (86 °F) and chlorine or ammonia levels are low, use a half dose.

Adding directly to the aquarium: If adding directly to aquarium, base dose on aquarium volume. Only do this if you cannot add to new water first—for example, if you're treating an emergency spike. Measure based on your entire tank size, not just the water you're adding.

6. Safety and storage

Prime is non-acidic and will not impact pH. It is safe for all fish, invertebrates, corals, and plants. There is no risk of overdosing in normal use. Store the bottle at room temperature in a dark place. The 100 mL bottle will stay fresh for several years with normal use.

What you notice and what to check

Your care log

Tank size: ______ gallons

Tap water type: Chlorinated / Chloraminated / Unknown

Standard dose per water change: ______ mL

Water change frequency: Every ______ days / weeks

Last dose: Date ______ Amount ______ mL

Notes: ________________________________________

Sources

Checked October 8, 2026. Reference addresses are plain text.

1. Bulk Reef Supply, "Prime – Complete Concentrated Water Conditioner – Seachem", https://www.bulkreefsupply.com/prime-complete-concentrated-water-conditioner-seachem.html
2. Seachem Laboratories, "Prime", https://www.seachem.com/prime.php
3. Aqua Forest Aquarium, "Seachem Prime Water Conditioner", https://aquaforestaquarium.com/products/seachem-prime-water-conditioner
4. Live Aquaria, "Seachem Prime Water Conditioner", https://www.liveaquaria.com/products/seachem-prime-water-conditioner