Crucians are adaptable fish and almost any still water will suit them.
This is a small pond from which you can net and stock fish into another, perhaps your main, lake or pond. See below. It does not have to be very big. It could even be as small as a regulation field pond, say 20m x 20m. If you have the place and the money, you can dig one of your own, not necessarily a hugely expensive exercise. If you do that, give it a year to mature before stocking with your brood fish.
Crucians don't need to be very big to spawn. Ideally you're looking for 3- or 4-year old brood fish and if they come from a crowded water these will probably not exceed 8". Remember, though, that not all 8" crucians will be as old as 3-4 years!
Crucians are not very easy to sex. You need an original stock of 20 or so fish to give you an odds-on chance of having both sexes present for spawning. If the pond is tiny, don't complicate things by putting in any other species.
In your nursery pond, you need to produce crucians big enough to avoid the dominant predators in your main pond or lake. If crucians are going to do well in your nursery pond you will probably end up with too many within a year or two. These may well be not big enough to survive the predators in your fishery. To increase the size of your crucians, thin them out by netting and/or feed during the growing months (April to October in the south of the country). "Thinnings" can go into your main pond anyway; there is nothing to lose though the odds against their surviving are high. Alternatively, you can sell them.
You must ensure that your brood stock is genuine, otherwise the whole exercise will be a waste of time and money. Don't just take the word of a dealer - he or she may not know!
If you are very fortunate you may have the chance to stock a bigger pond or lake that contains no other fish. Producing good crucian fishing in a pond of, say, half an acre or more is not too difficult as long as you follow a few simple guidelines.
The best fish to stock with crucians in a virgin pond are tench. The two species do compete to some extent because they feed in the same areas and on the same organisms but they don't hybridize and they don't seem to adversely affect each other's spawning, or if they do, because the crucian usually spawns before the tench, the scales are tipped in the crucian's favour.
If you want to go for another species, to give some variety for angling, then choose chub, perhaps rainbow trout (if the pond is suitable) - because in normal circumstances they won't spawn and so you can manipulate the numbers more easily.
Where they are the only species, or even in the company of tench, crucians usually spawn prolifically and within a year or two can over-populate the water. In this case they will remain very small, though apparently this doesn't discourage them from continuing to breed. In such a pond, maintaining numbers is usually not a problem - the challenge lies in keeping the fish growing and in good condition.
To increase the size of freely-breeding crucians in a pond or lake like this, you need to crop out small fish regularly, by netting or trapping. Please note that you need EA permission to do this legally. Ideally, you can use these fish to top up stocks in another, mixed species lake or pond.
In a virgin pond, then, to produce good crucian fishing (i.e. good numbers of medium-sized fish up to 2lbs) then avoid roach, rudd, pike, perch and carp - goldfish, too, of course, and any crucian hybrids.
In larger waters crucians grow surprisingly fast and to a good size, though a 3-pounder remains an exceptional fish. If they have to compete with other species, though, even apparently non-threatening ones like roach, numbers will steadily decline, because annual recruitment is usually in adequate: i.e. most of the young don't survive to adulthood.
The secret of successful management of crucians in a water of this kind is to stock regularly with fish of the right size. This means that you will have to buy in fish, which will be expensive, or produce them yourself by providing a nursery pond as outlined above.
It isn't very much use to buy in a small number of crucians and expect them to become established in your mixed population lake. One-off stockings are seldom successful. They will almost certainly disappear, apart from the occasional big fish. This is because old age, perch and pike and other predators like otters, herons and cormorants will gradually reduce the numbers of stock fish, and roach and rudd will ensure that crucian fry will not survive. Both of these latter species, and perch too, spawn earlier than crucians and their prolific fry either eat the crucian larvae or ensure that little food suitable for later baby fish remains.
So, any stocking with crucians has to be regular and with fish of a size that will make them safe from predation by the other fish in the pond - you can't do much about bird and mammal predators, I regret to say. For example, if you stock 2" - 3" crucians in a pond with a population of ½lb perch, then you've wasted your money. You might get away with 4" - 6" crucians, unless you have pike or very big perch there, but they will be more expensive and the stocking ideally needs to be repeated in the next two consecutive years. In that way you have a better chance of establishing a balanced population of crucians, with fish of more than one year class.
Even in a pond with no fish predators, several repeated stockings will give you a better chance of providing good crucian fishing. Remember that you cannot count on them to increase in numbers from natural spawning in the presence of other species, even apparently benign ones like roach and rudd.
In a water containing only carp and crucians, of course, the smaller species will struggle to compete and hybridisation will be a problem. It is not a mix that I would recommend, unless numbers of carp can be controlled. In some waters, carp do not spawn successfully and there decent crucian fishing can be produced by regular stocking. Ideally, regular nettings will eliminate crucian x carp hybrids before they become a problem.
There is one advantage to having predators in a crucian water. Scientific experiments have shown that crucians respond to the presence of predatory fish like pike and perch by quickly developing deeper bodies and higher backs. In time this change can become genetically fixed, resulting in the creation of a distinct "strain" of crucian such as the Yateley fish. Interestingly, it seems that if fish of this shape are transferred to another water without predators the high back is retained until conditions within that new water impose body shape change.
The disadvantage of a crucian/predator mix is that the crucians will be much scarcer and more difficult to catch. Sometimes they seem to disappear from the fishery, either because the few that remain avoid being caught or because they change their feeding habits to avoid predation - for example, feeding only very late at night, though experience has shown that this can happen for other reasons as well.
Crucians can live for at least 20 years in the wild, in my experience. Repeated stockings will therefore build up not only a balanced population but also give long-term sport.
I repeat that it is vital that you ensure that your brood fish are genuine crucians. Fish dealers do not always know, amazing as that seems, so it is up to you to scrutinize every fish that you put into your pond or lake. Go back to the descriptions and pictures on this site or send me photographs - they must be good and show fin shapes and scale counts.
If it is at all possible, in both types of fishery, net regularly to check on stock levels and proportions. Sell any surplus - there's always a market for good quality tench and genuine crucians.
A NOTE OF CAUTION: Do make sure to notify the EA and explain the situation. They will let you know what formalities (not very onerous) that you need to go through to do all this within the rules. Remember that it is a legal requirement to obtain permissions to net and stock fish. Check the EA website for details.