The first question you have to ask then, is what species?
To a large extent, here in the UK, there is a common answer and it's likely that you want to keep the Red Belly Piranha (Pygocentrus natteri). This species is by way and far the most common species seen in the UK, other species are available from time to time, however these are generally only held by specialist 'odd ball' shops, and then only with great infrequency.
The rules for keeping piranhas however apply equally to most species with a few common rules and so the description here will apply mainly to keeping Red Belly's, but is applicable as a starting point to all other species (particular reference to keeping the other species can be found with the species descriptions).
How many will you want to keep?
Obviously single specimens will require less space than multiple. However red belly piranhas (and all other Pygocentrus sp.) look much better and settle into aquarium life easier if kept in groups of at least 4 individuals.
The Aquarium
What size tank can you house? Piranhas need big tanks! They are messy fish that are quite active, and on the whole grow to a large size, the red belly, will if given proper care grow to more than 12" in length, other Pygocentrus sp. Will grow to more than 15" and some of the Serrasalmus sp. grow to more than 2ft in length. Get the biggest tank you can. Gifts of chocolate, flowers, car cleaning and weekends away are all useful tools in the securing of big fish tanks.
Piranhas grow quickly , red belly juveniles will grow from 1" to 6" in less than 1 yr with proper care, from then on they will grow approximately 1 to 3" per year depending upon species and conditions.
The best aquariums are the normal rectangular type. At a very minimum, one red belly piranha could be kept for its life in a 3ft standard aquarium (normally 12" wide and 15" deep here in the UK). A group of juveniles could be grown on in an aquarium this size for a few months, however, overcrowding leads to fighting, reduced growth rates and fish that are more susceptible to stress related diseases.
The minimum size aquarium for a group of 4 red belly piranhas would be 4ft long, 18" wide and 18" deep . This may seem like a large aquarium, but, as a general rule, piranhas need approximately 20 gallons of water per fish. This size aquarium will hold approximately 70 gallons (so barely up to size) and weigh nearly 850lbs when full.
Obviously the bigger the aquarium, the quicker the fish will grow and the more naturally they will behave.
1. Pygo species do …. Most Serrasalmus species have much slower growth rates!
2. It is sometimes possible to keep piranhas in two's or three's but invariably one fish will become dominant and eventually kill the others, keeping four disperses aggression sufficiently to ensure that all will bare only a limited amount of the aggression.
Water Quality
Because of the predatory nature of piranhas they tend to be fed meaty / fishy foods, they also have very healthy appetites and an ability to gorge themselves regularly. These habits will cause rapid deterioration of water quality, unless husbandry is top class.
It goes without saying that any uneaten food should be removed soon after the piranhas have finished eating, the last thing that you want is a smelly polluted tank, which is not good for either the fish or the keeper.
All piranha species come from South America, most from riverine environments (there are secluded lake based populations of many species, although these are difficult to find in the aquarium hobby). This natural tendency for flowing water should be replicated in the aquarium. All species benefit from good circulation in the aquarium. This, of course, is linked to filtration.
As would be expected, filtration for any piranha aquarium should be strong and effective. Most piranha keepers utilise power filtration, usually in the form of internal or external power filters . The best filtration is external canister type (the most popular makes being Eheim and Fluval, although there are several cheaper alternatives); these combine biological, mechanical and chemical filtration into one tidy unit. There is also the added advantage of not having to disturb the aquarium whilst undertaking filter maintenance. As a general rule the total volume of the aquarium should pass through the filter at least 4 times an hour, where groups of piranhas are being kept, this should be increased.
The return outlets to the aquarium can be used to create a current. Additional current can be provided through the use of power-heads placed within the aquarium. Piranhas kept in aquarium with a strong current tend to be better toned, fitter, more active, more aggressive and grow more quickly than those kept in aquarium with very little current.
The water itself should be maintained at a temperature of between 78 and 82 degrees F. This is simply achieved through the use of standard combine heater / thermostats. As a general rule heaters should be provided at the rate of 4 watts per gallon, therefore a 70 gallon aquarium will require approximately 280 watts of heater …a 300 watt heater would suffice. It is also worth considering that piranhas are large and aggressive and can break aquarium decoration, including heaters, for this reason it is worthwhile using two lower wattage heaters rather than relying upon one of the correct wattage.
In their natural habitat piranhas are at home in relatively soft water with a ph approaching neutral at between 6.5 and 7.2. However in the UK most piranhas encountered will be captive bred, either in the Far East or in Holland. These are much more hardy specimens and are perfectly at home in average UK tap water (with its PH of approximately 7.5) with moderate carbonate hardness.
3. Although under gravel filtration could be utilised, or used to supplement power filters, it is generally not recommended as the primary filtration system for piranha aquariums
4. With the addition of substrates such as charcoal
5. Water quality does vary greatly throughout the UK, however, usually your local fish store will be keeping fish in similar conditions to those that most of their customers will experience, so a general rule is that if the store can keep them healthy, then you shouldn't have too many problems.
Lighting
Piranhas are happiest in the dark! Obviously this is not best from and aesthetics point of view, so a normal grolux 'type' light will suffice, although piranha will be quite relaxed in a dimly lit tank.
Normal aquarium florescent tubes are the best, if you want to stimulate plant growth, then one (or multiples of) of the triton type tubes will be suitable. Remember the lighting is for you, not for the fish.
The fish will usually acclimate to brighter lighting, although will often remain skittish under very bright conditions.
Decoration
The aquarium substrate should be inert, most commonly used is a silica / quartz based gravel. The grade is irrelevant as piranhas are not diggers and rarely disturb the substrate. Another option is the use of an aquarium grade sand, this is becoming more easy to find and has the benefit of being easy to clean, as any dirt is just siphoned from the surface, however it should be noted that it is unsuitable for use with under-gravel filters.
Dark substrates tend to show off the fishes colours best, with black being particularly effective.
Under no circumstances use crushed coral sand or gravel or crushed seashells, the calcium / carbonate content is too high.
Piranhas on the whole are fairly active and for this reason the aquarium should be uncluttered, although the use of bog / mopani / curio wood, and plastic and real plants will make the fish feel at home.
Any inert rocks can be used as decoration in the aquarium, although sharp edges should be avoided, once again stay away from calcium based rocks such as tufa and limestone.
its ust not working says theres an error 
to pfuk, there are a few members on here from your area, I'm sure they'll steer you in the right direction.