A Short Guide to Fish-In Cycling

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Are you interested in a fish-in cycle for whatever? Perhaps you had a fish thrust into your care and have no choice. Or maybe otherwise.

Whatever the case, I do not judge. Instead, I am here to help.

So here is the guide on how to perform a fish-in cycle.

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Here we go.

1. Do not rely on Seachem Prime or similar products to detoxify ammonia or nitrite. Evidence suggests that contrary to the manufacturer's claims, Seachem Prime neither detoxifies ammonia nor nitrite. However, it is still suitable as a dechlorinator. Most likely, similar products claiming to detoxify ammonia/nitrite probably do not do so either.

2. Rely on water changes to keep your livestock safe. Ammonia/nitrite/nitrate does not need to spike when performing a fish-in cycle. The theory behind such suggestions is that the nitrifiers ('beneficial bacteria') need ammonia/nitrite to grow and reproduce. While this is true, your fish will be producing more constantly. So even if you do a 100% water change and eliminate all the ammonia/nitrite, there is zero concern about slowing the cycle. Your livestock will soon produce more.

3. The frequency and amount of water change you need to do varies. Some guides will tell you that any amount of ammonia or nitrite is immediately toxic above zero. This is false. Let's start with ammonia. Ammonia is measured as total ammonia by test kits such as API. The concentration of total ammonia that is actually toxic to fish depends on pH and temperature. Here, check out this guide. Yes, it is an external guide, because it is really good, so I see no need to re-write it. Do let me know though if the link no longer works. Anyways, the long and short of it is that the lower the pH and temperature, the higher the total ammonia concentration needs to be to be toxic. For example, at a pH of 7 and temperature of 25 degrees Celcius, even 4ppm of (total) ammonia is not toxic! Shocking, I know. But hey, that is also why sometimes products like Seachem Prime seems to work, when in reality total ammonia was simply not high enough to be toxic at that pH and temperature. So you just need to do a water change when the concentration comes close. Generally, a 25% water change works, but 50% works too, especially if the toxic level is lower. Frequency depends on how fast your tank gets to the toxic point. As for nitrite, there is no specific guidance, but generally try to keep it below 2ppm is best. If your livestock shows any signs of nitrite toxicity, do a water change right away, and bare in mind that level of nitrite as something you need to avoid. By the way, salt (NaCl) helps alleviate nitrite toxicity.

4. You may not need to keep your livestock in a separate temporary container while cycling the actual tank. Unless said temporary container is already cycled, there is no point. After all, your livestock would be in an uncycled environment anyway; they might as well be in their actual tank. They will still produce the same amount of ammonia.

5. Your fish-in cycle is done when both ammonia and nitrite can consistently measure zero. That's it. Again, you DO NOT need to see either hit a particular mark. You do not even need to see nitrate. Here's the idea. If you are feeding your fish (or other livestock), and ammonia and nitrite consistently remain at zero without having to rely on water changes, then the tank is cycled for the current bioload. The keyword is bioload - i.e., the current stocking of the tank. Or rather, the current feeding amount for the tank.

6. The way to add more fish is to slowly ramp up feeding. Even with more fish, if you feed the same, then the amount of ammonia/nitrite produced would remain more or less unchanged. It is when you increase the feeding that more ammonia/nitrite is produced. So just ramp up feeding slowly.

7. You can speed up the cycling process with established biomedia or a bottled bacteria product. However, not all products are good, or are even suitable for cycling. Seachem Stability and similar products for example, are entirely unsuitable for cycling and should be avoided. Currently on the market, the best product is FritzZyme TurboStart 700, followed by its less concentrated counterpart, FritzZyme 7. Tetra SafeStart(+) is also a reputable product. Moving water from a cycled tank over does not really help much, as there is a negligible amount of nitrifiers in the water column. Squeezing gunk from filter media may not help either, as the gunk can either be just gunk, or comprised of microorganisms that are not nitrifiers. Of course, there can be nitrifiers in the gunk, but it is more effective to move the biomedia itself.

Tada!