How to Cycle a Tank with Seachem Prime Conditioner
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A fish in cycle with Seachem Prime means you keep fish in a new tank while the beneficial bacteria build up, and use Prime to make the ammonia and nitrite those fish produce less harmful in the meantime. According to Seachem’s own product page, Prime binds ammonia, nitrite and nitrate so they are harmless to fish but still available to the biofilter bacteria, and it does not stop the cycle. This guide walks through what the page says, how the numbers fit together, and where it stops giving instructions.
Quick answer
- Prime detoxifies ammonia, nitrite and nitrate and removes chlorine and chloramine, but it does not remove them from the water.
- The bound compounds stay available to bacteria, so the cycle still happens.
- Seachem’s page says binding lasts up to 48 hours; re-dose every 24 to 48 hours, or every 24 hours if ammonia or nitrite is rising within a day.
- The page gives no step-by-step fish-in schedule, so test often and ask an aquarium-experienced vet if a fish looks ill.
What the cycle is
A new tank has no established bacteria to handle fish waste. Fish release ammonia. One group of bacteria turns ammonia into nitrite, and another turns nitrite into nitrate. The Wikipedia article on the nitrogen cycle describes these steps, naming Nitrosomonas for the first and Nitrobacter for the second, and notes that ammonia is highly toxic to fish. The fishkeeping article on Wikipedia adds that new tanks suffer from “new tank syndrome” until they mature, and that adding too many fish too quickly can cause ammonia stress. The point of cycling is to let those bacteria grow to a size that can process the waste your fish make.
A fishless cycle does this with no fish present. A fish-in cycle does it with fish already in the water, which is why the toxicity of ammonia and nitrite becomes the main concern.
What Seachem says Prime does
From the Seachem Prime page:
- It removes chlorine and chloramine from tap water.
- It detoxifies ammonia, nitrite and nitrate by binding them, so they are harmless to fish but still available to the biofilter bacteria.
- It detoxifies heavy metals present in tap water at typical concentrations.
- It is non-acidic and does not change pH.
The page states that Prime may be used during tank cycling to reduce ammonia and nitrite toxicity, which lets the biofilter work more efficiently, and that it does not stop the cycle because the bound compounds remain available to the bacteria. It also says to use it at start-up and whenever you add or replace water.
Dosing as stated on the page
| Situation | What the page says |
|---|---|
| Standard dose | 1 capful (5 mL) per 200 L (50 US gallons) of new water |
| Small volumes | Each cap thread is about 1 mL |
| Where to dose | Adding to new water first is preferred; if dosing the aquarium directly, base the dose on aquarium volume |
| Very high chloramine | A double dose may be used safely |
| Emergency nitrite detoxification | Up to 5 times the normal dose |
| High water temperature | Above 30 C (86 F) with low chlorine or ammonia, use a half dose |
| Re-dosing | Every 24 to 48 hours; every 24 hours if ammonia or nitrite is rising within 24 hours |
The page also says that the emergency dose of up to 5 times is safe when ammonia or nitrite is above 2 ppm. Read the current label on your bottle before dosing, because the product page is the manufacturer’s summary and the label is the instruction you should follow.
Putting it together: a fish-in routine
Seachem’s page does not publish a fish-in schedule, so the following is how the stated facts fit together, not a tested protocol.
- Treat new water before it goes in. The page prefers adding Prime to new water first. This handles chlorine and chloramine before they reach the fish.
- Test ammonia and nitrite regularly. The cycle is complete only when the bacteria can turn both to nitrate, and your test kit is how you see that.
- Re-dose on the page’s timetable. Binding lasts up to 48 hours, so a tank that is still producing ammonia needs repeat doses. If readings climb within 24 hours, the page says to re-dose every 24 hours.
- Change water when readings are high. Replacing water removes some of the load, and the page says to use Prime whenever you replace water.
- Add bacteria if you want to support the cycle. Seachem’s separate Stability page says fish can be added at any time as long as dosing continues for the full 7 days, and that a full cycle takes about 4 to 6 weeks, since timing varies by tank.
The test kit caveat
Seachem’s page notes that Nessler-based liquid test kits read falsely high when Prime is present. It recommends Seachem’s MultiTest Ammonia kit, or waiting, since Prime dissipates within about 24 hours. If your ammonia number looks alarming right after dosing, check what kind of test you are using before you assume the worst, and check the instructions that came with your kit.
How Prime and Stability fit together
The Stability page describes a bacterial product that helps establish the biofilter quickly and helps prevent new tank syndrome. For a new tank it lists 1 capful (5 mL) per 40 L (10 US gallons) on day 1, then 1 capful per 80 L (20 US gallons) daily on days 2 to 7. For ongoing use it says to dose 1 capful per 80 L when adding new fish, medicating or doing a water change, and monthly or with each water change for routine maintenance. It also says to turn off UV or ozone units during dosing, and for at least 24 hours after adding fish or doing a water change. It does not use the phrase “fish-in cycle” but does cover adding fish during dosing, and recommends AmGuard for emergency ammonia control. Treat these as manufacturer directions and check the bottle.
Limits of what the sources tell us
- The Prime page does not give a fish-in schedule, a feeding amount, or a safe ammonia ceiling for any particular species.
- It does not say how many fish a new tank can hold during a cycle. The Wikipedia fishkeeping article only advises adding fish gradually and monitoring water quality.
- Nothing here is medical advice. If a fish is gasping, lying on the bottom or has damaged fins, ask an aquarium-experienced veterinarian.
Related reading
If you are comparing methods, see our guide on how to speed up a fish tank cycle, and for the basics of the conditioner itself, how to use Seachem Prime. Our article on using Prime and Stability together covers the pairing in more detail.
FAQ
Does Seachem Prime stop my tank from cycling?
No. The page says the bound ammonia, nitrite and nitrate remain available to the biofilter bacteria, so the cycle continues.
How often should I re-dose during a fish-in cycle?
The page says every 24 to 48 hours, because binding lasts up to 48 hours. If ammonia or nitrite is rising within 24 hours, it says to re-dose every 24 hours.
Why does my ammonia test read high after dosing?
The page says Nessler-based kits read falsely high with Prime present, and suggests Seachem’s MultiTest Ammonia kit or waiting, since Prime dissipates within about 24 hours.
Can I overdose Prime in an emergency?
The page says up to 5 times the normal dose may be used for emergency nitrite detoxification and that up to 5 times is safe when ammonia or nitrite is above 2 ppm. Follow the bottle label for your product.
How long does a full cycle take?
The Stability page says to expect about 4 to 6 weeks, since timing varies by tank. The Wikipedia fishkeeping article describes slow growth over 6 to 8 weeks as one approach.
Bottom line
Use Prime to make the waste in a cycling tank less toxic, not to skip the cycle: dose on the page’s 24 to 48 hour timetable, test with a kit that works alongside Prime, and keep stocking light.
