Stocking After Cycling

I have read a good deal about fishless cycling with ammonia, both here and on other sites. However, I find some of this inconsistent and confusing, particularly with regard to stocking after the cycle is complete. There seem to be three conflicting ideas :

1. The ammonia cycling process should provide enough bacteria to deal with a full stocking of fish for that tank
2. It's safe to add up to two-thirds of your total stocking level immediately
3. You should still only add a few fish at a time.

The third seems nonsense, why bother to spend several weeks building up a good number of bacteria to then only add a few fish? Surely this means that the number of bacteria will decline until you will be back to a similar situation as if you had started with a fish-in cycle? However, a lot of people seem to say "I've finished cycling my tank and now I've been out and bought a few fish to start stocking". I also wonder where the two-thirds idea came from. Is it just a guess? Is there any reason why, after a cycle is complete, one should not add all of the fish in one go? The typical shop fish is, I should say, about 60% of its final size and so one would actually only be adding 60% of the final load. Should the filter bacteria after a full ammonia cycle not be easily sufficient to cope with this?

I can see definite advantages in adding all the fish at one time, not least that it means less trips to your favourite shop, 35 miles each way for me. In addition, every time new fish are added there is the possibility of introducing disease. By adding all fish at one time, you can keep a close eye on them for a few weeks, cure any problems and, if you don't add more fish later, there is no chance of bringing disease in. Finally there is the subject of “community relations” in the tank. Quite a lot of queries on forums are of the type “ I had x, y and z in my tank and they all got on well. Now I've added w and everything is chaos, even though these are all supposed to be peaceful community fish”. I've observed that even the most peaceful of fish can be quite territorial. If fish are added all together, they seem to sort out their relationships and establish their “personal space” quite amicably, especially if they are still immature, but once things are settled new fish seem to often unsettle them again.

So the question is, is there any reason why one should not add at one time to a properly cycled tank all of the immature fish which will eventually grow to reach the full stocking capacity of that tank?

Just sticking with the three options:

Provided you have cycled with 3ppm ammonia (which is enough to support a properly stocked tank) -

Option 3, only a few fish. The people who say that probably don't understand the difference between fishless and fish-in!

Option 1, fully stock the tank. Theoretically, this is the correct answer. But the danger is knowing exactly what fully stocked means. A lot of newcomers grossly overestimate what their tanks can hold. After all, the tank looks empty with all those tiny fish. As long as a 'proper' stocking has been worked out in advance and the list adhered to, fully stocking the tank is OK. But, see next comment.

Option 2, get two thirds of the total list. This is probably a bit conservative, but I tend to prefer not stocking 100% as soon as the tank has cycled, maybe something like 75 - 80%.
Cycling grows the two species of bacteria in the filter, but not all the other micro-organisms that eventually grow in the tank, other organisms which help keep the water conditions stable. A tank is considered mature after 6 months and before this is at risk of conditions suddenly becoming unstable. If this should happen, a slightly understocked tank would cope better.
Some fish do not do well in immature tanks. Neon tetras are the often quoted example, but fish like rams also do much better in a mature tank. If your list contains any fish that need a mature tank you need to wait before buying them, but still leave room for them when the tank has matured.


Adding all your proposed fish purchased at the same time from one shop will indeed reduce the likelihood of introducing a disease. Since most shops use a central filtration system, all the fish in the shop will be exposed to the same diseases. The potential for introducing a disease with a subsequent batch of fish, a disease that the fish already in your tank do have resistance to, is reduced.


It is quite possible that the chosen shop will refuse to let you buy all those fish at once, unless you lie and tell them your tank is 10 times bigger than it really is or that you have 10 tanks and want a few fish for each tank! . Telling them it is OK as you have just finished a fishless cycle will only get you puzzled looks. After all, everyone knows you set your tank up and get a few fish to start the cycle, getting so many at once is just dangerous. Other shops have a very inflexible policy of refusing to sell more than x fish to any customer under any circumstances. Faced with these types of shop, you have no alternative but to visit several in order to fill your list or to go back to the same shop at weekly intervals till you have all your fish.
A shop may not carry all the species you want, and you may not be able to visit enough shops on the same day to buy all the fish on your list. In this case you may have to wait another week or so to complete the list.
In these cases, you won't be able to get 100% stocked on the same day. We now know that the bacteria will initially become dormant rather than die off straight away if there is insufficient food, so stocking over a week or two isn't a big problem. But if your fish choices mean that you have to wait six months before getting the final fish then some bacteria will have died off and the last fish must be added carefully.



Juvenile animals don't produce less waste than adults in proportion to their size. Growing animals produce more waste than their size would indicate. A fish that is 60% of it's final size will produce more than 60% of the waste of an adult fish; somewhere between 60 and 100%.

It will be interesting to hear how your tanks fare once you have fish for them.

You may have 10 tanks, but if you wanted to buy enough fish to fully stock them all at once the shop would either think Christmas had come early or they'd go into shock at the very idea 

As an ecologist you will understand about creating a whole system of micro-organisms and their inter-dependance. With heavily planted and second hand tanks that are supposed to have cycled filters, dosing with ammonia is the way to prove they are safe for fish. If the tank can remove 3ppm ammonia and any subsequent nitrite both to zero in 24 hours, it is safe for fish. It doesn't matter is it is the plants or the filter bacteria removing the ammonia, so long as it is removed. The only danger comes if you are as useless as me at growing plants and the whole lot suddenly die, removing a good chunk of the ammonia removing system.

Buying on-line is fine as long as you bear in mind that your water could be very different form the supplier's. I would ask them for their hardness and pH, though they might not tell you. Even if the water is quite similar, it would be worth drip acclimating them to be on the safe side.