Rescuing A Fish In Cycle Gone Wild - Part I

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Fish Connoisseur
Dec 31, 2004
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Part I

If you are reading this it means you have tried to cycle your tank using fish and have run into problems and don’t know what to do. Before we begin to discuss the potential solutions, we would like to make it clear that we do not recommend to anybody, and especially anyone who is relatively new to the hobby, that they cycle a tank using fish as the ammonia source. (More on this below.)

Before we offer specific help, it is important for you to understand that Cycling with fish requires that one learn and do a great deal more than is needed to do an easier, faster and fish safe fishless cycle.


There are three basic types of fish in cycling situations regarding one’s stocking level:

1. It is potentially possible to complete the cycle without harming or killing fish.

2. It is likely not possible to complete the cycle without harming and/or killing some fish.

3. It is not possible to complete the cycle and trying will likely harm and/or kill the fish.

We do not wish to encourage anybody to cycle with fish, so most of what follows is not designed to assist people in trying to continue with a normal type fishless cycle over several weeks. However, it does suggest other alternatives which can effectively remedy your situation.
If your tank is in situation #2 or #3 above, you will have to remove some number of fish in order to proceed in a manner that is fish safe and should solve the problems you are experiencing. You will have to remove enough fish to get your tank into situation #1 above.


The test readings used in this article are for API and similar type test kits which measure in total ions. Some kits will measure using a different scale, they are only measuring the nitrogen ions. You can tell when a kit reads just the nitrogen ions by the way they state things. The typical kits will say they measure Total Ammonia (NH3 + NH4), Nitrite (NO2) or Nitrate (N03). Kits that measure only the nitrogen ions will usually say they measure Total Ammonia-Nitrogen (NH3-N + NH4-N), Nitrite-Nitrogen (NO2-N) or Nitrate-Nitrogen (NO3-N).

Just like one can convert distances between miles and kilometers, one can convert between the total ion scale and the nitrogen ion scale. Since this article is using the total ion scale, if you have a kit that reads in –Nitrogen (-N), you must multiply your results as follows:


One of things hobbyists learned over the years was something science has researched as well. When it comes to sensitivity to ammonia, nitrite and nitrate, not all fish are the same. Some fish can handle higher levels than average and some can’t handle almost any at all. Some fish handle more ammonia than nitrite or nitrate and others are the opposite. So the one thing old time fish keepers learned to do was to start off with fish that were hardiest in terms of the cycle.

The odds are good that if you are reading this you were completely unaware of the above facts and have likely not selected the more hardy cycling fish and probably have some of those which are at greater risk from it. Moreover the size and age of fish can also be a consideration.

Finally, most any invert (shrimps, snails etc.) are usually very intolerant of any of the big three toxins related to cycling. If you have these you definitely need to think about getting them out of the tank until it is made safe for them.

In order to know what stocking situation you are in based on the number and kinds of fish you have, you may need to start a post in the cycling section of the forums in which you will need to post your specific information: tank size, fish types and numbers and other pertinent cycling information for your tank.


In a fish in cycle out of control the problem is really very simple- the fish are producing too much ammonia for the amount of bacteria in the tank. The bacteria cannot reproduce fast enough to prevent the level of ammonia from harming fish. And then the nitrite problems will begin.

The ideal solution is simple, get the fish out and convert to a fishless cycle.

When this is complete you can add all the fish back, and likely can add even more than you removed.

The next best solution is to reduce the fish load to a level where you can add sufficient bacteria to get the tank fully, or very close to fully, cycled instantly or close to it.


Rehoming fish will help a lot and it is really the only option for having any control over ammonia production. Fewer fish make less ammonia. There are several ways you can temporarily rehome some or all of your fish in order to complete the cycle rapidly and safely.

Return them to the store: One good option is to return the fish to the store where you originally purchased them. Some stores may not want to take back fish, especially if it is for cash refund. However, many places will take them back for store credit. If you explain to the shop that you are not looking to get rid of the fish and that you will want them back once you resolve the cycling problems, then many stores are often willing to cooperate. It never hurts to ask in these cases.

Park them with other fishkeepers: Another way to rehome fish is with other fish keepers. You may need to get help from a couple of folks to place all the fish for a short time. If you do not know anybody near you, you can try posting on this site for help and may find it. There are also local fish clubs you might find near you and they may be able to help with the problem as well.

Park them in another container: Finally, you can use a temporary uncycled container to house the fish while completing a fishless cycle in their original tank. This process takes more work as you will have to perform water changes anywhere from every other day to daily. You can use almost any clean container that holds water- Rubbermaids are great for this. You will also need an air pump and sponge filter or decent air stone, a thermometer and a heater. Alternatively, a hang on or internal filter will do. The goal is to get water movement and surface agitation in the container. It can also help with mechanical filtration. You do not need to have a cycled filter. Water changes should be performed as unobtrusively as possible to minimize the stress to fish. Also feeding should be minimal, every few days should be fine. Finally, one can throw in plants, especially floating plants, which will consume ammonia.

You will be able to get the main tank cycled for a full fish load a lot faster than you can complete the fish in cycle. When the main tank is done not only can your return the parked fish, but you can add more fish to be fully stocked. The one risk is the additional fish will not have been quarantined by you before going in. If you opt for this method, you should then follow the site’s directions for fishless cycling found here:


In a fish in cycle out of control the problem is simple, even with a “manageable” fish load there are not enough bacteria to process all the ammonia to nitrite and all the nitrite to nitrate! After all, the goal of cycling, with or without fish, is to build up bacterial colonies of sufficient size to handle these things in your tank. Neither waiting for nature to take its course (i.e. a fish in cycle) nor doing repeated large waters changes for an extended period of time is really a viable solution. Either may still cause harm to fish.

The best alternative at this stage is to Add Bacteria.

This means adding enough, or pretty close to enough, bacteria to the tank to have it become fully cycled. You can do this in two different ways:

1- Bottled Bacteria- There are a couple of brands this writer suggests- Dr. Tim’s One and Only Nitrifying Bacteria or Tetra’s Safe Start.1 (Follow their directions to the letter.)

2- Material from a Cycled Tank- This would be donated from other fish keepers and/or stores willing to do so. (USA members can buy cycled media here:

The two different methods can be combined, that is, one can use bottled bacteria and seeding from established tanks together to reach a needed level.


While bio-media from a cycled tank are likely to hold high concentrations of the desired bacteria to put into a new tank, they are is not the only source. In any tank the bacteria will colonize any and all of the best places for them to live. Because the bacteria attach themselves to hard surfaces via a bio-film they create, they cannot actively seek out food, it must come to them. Therefore, the best places are those with good circulation to bring them ammonia/nitrite/nitrate oxygen, carbon etc. They are also light sensitive, so they tend to be in places not in direct light. The following will all bring in bacteria/cycling help (they are listed in order from most to least helpful):

Cycled Bio-Media-

Under normal conditions, in an established tank, the largest concentration of nitrifying bacteria should be found in the bio-media inside the filter. This includes such things as sponges, ceramic noodles, bio-rocks, bio-balls etc. It is best not to remove more than 25% of such media from a cycled tank as it can result in a cycling related spike that can be a problem for the established donor tank. It is best to keep cycled media moist when transporting it

Live Plants-

These offer a double way to help. They will host some of the needed bacteria. But they will also consume ammonia (as NH4) which reduces the amount of bacteria it will take to keep a tank cycled. The more plants, the more help. If these are used as an emergency measure and you plan to remove them later, only remove one third of them at a time. Wait a week or so between removals. This prevents an ammonia/nitrite spike due to removing too many at one go. Live plants also consume nitrate, another bonus in an established tank.

Substrate from a Cycled Tank-

Some amount of bacteria colonizes the upper layers of the substrate in a tank. (To a depth of about ½ in. - 1.25 cm. if not planted) Thus taking surface sand or gravel from an established tank will bring in some amount bacteria. Smaller quantities can go into a bag in your filter or just drop the bag into the tank.

Decor from a Cycled Tank-

Some amount of bacteria also colonizes the underside surfaces of aquarium décor such as rocks, drift wood, caves, commercial decorations etc. So, taking some of these things from an established tank will help to bring in some amount of bacteria.
(Leave the fish behind J)

Filter Squeezings- from sponge type media taken from an established tank will put some bacteria into a tank. This looks quite messy but it really does help. The cycled media is simply “rinsed out” in water in the new tank. (Bits of the bio-film in which the bacteria live will be detached and end up in the new tank water and then find a home there.

Used Filter Floss- is usually a poor media for seeding a tank. Its purpose is to fill up with gunk which makes it a poor long term home for bacteria. It may be better than nothing, but it is of minimal help here. The filthier and more clogged it is, the less nitrifying bacteria it can support.

Note: If you cannot transfer any of the above things directly and need to park them for a bit, keeping them in dechlorinated tap or removed tank water is recommended to prevent drying out.


There are advantages and disadvantages for either method of adding bacteria:

- Bottled bacteria will cost more than + donated bacteria from cycled tanks which is usually free or sent for the cost of shipping.
+ Bottled bacteria will usually be easier to find than enough -
donated bacteria laden items from cycled tanks. It may take more than one source for this.
+ Bottled bacteria should be more accurate in terms of knowing what you add being sufficient to do the job. There is no way to know beforehand how much - donated bacteria you may be getting from other tanks. This does not mean one cannot get enough, you might even get more than you need.
+ Bottled bacteria should always be disease free, the same cannot be said of - donated items from cycled tanks.
- Bottled bacteria can be mishandled or out of date rendering it either dead or virtually useless. It is not a 100% guaranteed solution. + Donated bacteria should normally be live, especially if not shipped.
+ Donated bacteria will likely include some of the other non-cycling related bacteria which will also establish over time in a tank. - Bottled bacteria usually won’t have these other bacteria included.


It is important to insure proper conditions in your tank before adding bacteria. Both high ammonia and high nitrite can be harmful to bacteria. To avoid this problem test for both. For Dr.Tim’s (or for seeding from an established tank), if ammonia is near or over 5 ppm and/or nitrite is not clearly under 15 ppm do a 50% water change and add back water dechlored before it is added to the tank. Tetra Safe Start requires these things be even lower and advises one not to use dechlors etc. 24 hours before adding their product and for the next 2 weeks thereafter. Whatever commercial product you might use, be sure to follow their directions to the letter rather than those in the typical cycling article or for another product. (If you are using an API (or similar) nitrite test kit you may have to perform a diluted test as this kit tops out at 5 ppm. (Directions for doing this are in Part II of this article.)

Only dose dechlor at normal strength- Do Not Over Dose. Ideally, you want to do this the day before adding bacteria to the tank. Test for ammonia and nitrite again before adding the bacteria to be certain the conditions are OK and to know what the levels are. We also suggest that you buy more bacteria than you think you need if going this route. It is better to have too much than to have too little. The same applies to seeding bacteria taken from established tanks, too much is better than too little.


[sharedmedia=core:attachments:75259]If you have added sufficient bacteria to get the tank cycled, you should not be able to detect either ammonia or nitrite within a day or two. Therefore you will need to test for both ammonia and nitrite at 24 hour intervals. You may test at 12 hour intervals to see if things are moving faster and staying safe, but it is the 24 hour tests that matter.

For a tank without fish, a fully cycled tank should be able to process between 2 and 3 ppm of ammonia and show 0 nitrite as well, in 24 hours or less. If you have switched to fishless cycling, and you do not get 0/0 readings within 24 hours of adding 2 or 3 ppm of ammonia, follow these directions:

If ammonia and nitrite do not both read zero, continue to test daily. Whenever ammonia is at .25 ppm or less and nitrite is clearly under 1 ppm, add the same amount of ammonia and test in 24 hours. Follow this pattern of testing and adding until both tests read 0 ppm. The cycle should not take much longer to be completed.

For a tank with fish, a fully cycled tank should also test 0/0 for ammonia. If you have added bacteria to a tank with fish, you cannot add ammonia, but you must still test. If you are almost, but not quite, cycled, you may detect lower level readings for ammonia and or nitrite. Numbers should clearly be under 1 ppm for both ammonia and nitrite 24 hours after you have added bacteria. If they are not under 1 ppm, you have not added enough bacteria. Add more or remove some fish. You may also need to do a water change if the numbers are above 1 ppm or if they are rising.


If you are still not convinced to give up the idea of completing your cycle as suggested above and you think you want to continue on with fish, then we suggest you read Part II

1 These products are preferred as there is there is peer reviewed scientific research which has identified the nitrifying bacteria at work in fresh water aquariums. The two products mentioned contain those specific bacteria. The other bacterial starter products available do not contain both of these bacterial strains and some do not even contain any live bacteria at all. These two products have patent protection.

Ammonia oxidizing bacteria research
Nitrite oxidizing bacteria research
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