Before buying Piranha consider the size of the tank you will need for your piranha, the bigger the tank the better. I would recommend only keeping 2 piranhas per 60gallon tank (depending on species), and at least 20gallons extra for each additional fish. It is possible to add more fish, but if you want your piranhas to live happy and show natural behavior, this is the best formula, in my experience. Do NOT get any kind of 'tall' tank or hexagon style tank for them.
You should place your Piranha tank in an area of the room where it does not get direct sunlight, because the sunlight can cause the tank to overheat or get an algae bloom. You should also consider a low-traffic area of the room to place them in, because too many people walking by all the time can stress them.
Piranhas are very hardy fish and can adapt to a wide range of water conditions, but it is better and more healthy to target the ideal conditions for them. Piranhas do fine in a temperature from 75° to 80°, PH: 5.5-8.0.
Piranhas in their natural habitat usually live in murky water or in and around forest bottoms during the rainy season where not much light penetrates through the forest canopy. Piranhas eyes have adapted to these conditions, they have a large pupil (black part of the eye) that is designed to absorb light so that they can see better. However, unlike a humans eyes their pupils cannot contract to regulate the amount of light entering the optic nerve this is why you see your Piranhas go crazy when you turn on the aquarium lights. It hurts their eyes. I've experimented a little with my Piranhas to try and prove this point. I turned off all the lights in my room at night, and used a dim flashlight to shine in their eyes, their pupils did not react to the light. Therefore it must mean that Piranhas cannot regulate the amount of light entering their optic nerve, and obviously they do not have eyelids, so they couldn't just shut their eyes if light is too bright. This is why I do not recommend the use of aquarium lights.
Piranhas will eventually become used to aquarium lights (if you absolutely MUST use them), but they will always be VERY nervous and will fright easily. Long time exposure can also possibly cause dammage to their vision.
If you must use aquarium lights try using a thick bunch of floating plants to dim the brightness of the light as much as possible. Or view your fish by using the room light, it's more than enough lighting to veiw your fish, and it doesn't disturb your piranhas. Natural sunlight from a nearby window is also more than enough, and does not seem to bother Piranhas at all.
When keeping Piranhas its a good idea to use some type of tank furnishings to make them feel secure in the aquarium. Probably the best of these would be plants (plastic or real), driftwood, small rocks, and gravel. They create a natural appearance and places for your Piranhas to hide when frightened.
Plants, real and plastic:
If you choose real plants the best kinds would be hardy low-light plants, such as hornwort, java moss/fern, etc. or some types of floating plants. If you choose plastic, you can use whatever types you prefer, since they are not live plants they don't require any care. Or use a combination of live and plastic plants.
If you plan to use driftwood you must first soak it in a bucket of water until all the tanic acid leaches out of it, you'll know this is happining when the water in the bucket starts turning a brown tea color (Some Piranhas in their natural environment like this type of water, but it tends to have an un-attrative look in the aquarium). When the water in the bucket turns a brownish color empty it and re-fill it with clean water until it stops turning brown. After the water stops turning a brownish color this would mean that all the tanic acids have leached out of the driftwood and will be ready to use in your aquarium. If the piece of driftwood is too large to fit in a bucket try soaking it in a small kids pool, or in your bathtub, etc. Improvise.
With gravel I prefer to natural colored kinds, but you can use whatever you want as long as its made for aquarium use, and remember to wash it before using it in your aquarium. Personally I prefer to use barely enough gravel to cover the bottom of the tank, it makes cleaning alot easier.
When choosing rocks pick ones that are not too jagged or your Piranhas could hurt themselves on them, and always wash the rocks before putting them in your aquarium. Personally I prefer smooth rocks for my Piranhas. Also some rocks can raise the PH of the water which can be bad for your Piranhas. Here are some good and bad rocks:
The following rocks are ok for your Piranhas and will not affect your water chemistry: Granite, Basalt, Gneiss, Slate, and Quartz.
The following rocks are bad for your Piranhas that can turn the water hard, and akaline: Limestone, Marble, Dolomite, Calcareous sandstones, and any soft, chalky rocks.
Other tank furnishings that should be avoided: dead coral, crushed coral, and sea shells. These will raise the PH of the water even higher.
These types of furnishings you can use as hiding places for your fish, that are safe for the aquarium, but do not have the natural apperance: flower pots, and pvc pipes.
Filtration and other Equipment:
Before you start your Piranha tank you will need a good mechanical and biological filtration system, and other tank equipment. Here is a list of filters you can choose from, and a list of other equipment you may need.
These type of filters are good for filtering water through large volumes of media, but they sometimes have a low GPH (Gallons Per Hour) rate, they are also more versatile (most types) that allow you to use a wide range of media. But the major draw back on these filters is that they are harder to clean, and tend to be expensive.
These are good mechanical filters, and some even have a "bio-wheel" for excellent biological filtration. These filters usually hold a small volume of media, but make up for it in high GPH (Gallons Per Hour) rate. These filters are usually cheap and require frequent cleaning, but its not nearly as hard as cleaning a canister filter.
Under Gravel Filters:
These filters have satisfactory biological filtration, and very little mechanical filtration. If you plan to use this filter it should be in conjuction with another type of filter, I.E. power filter, etc. They require an air pump, or powerhead to run and at least 2" layer of gravel on top. These filters are cheap and best used for fry, or small species tanks.
These filters provide very good biological filtration and somewhat good mechanical filtraion. They require an air pump, or powerhead to run. These filters are cheap and best used for fry, or small species tanks.
Inside Box Filters:
These filters provide satisfactory biological, mechanical, and checmical filtration. They require an air pump to run. These are cheap and best suited for fry, or small species tanks.
These filters are the best all around filter. You can put anything you want in these filters. They provide excellent biological, mechanical, and chemical filtration. They do not need frequent cleaning, but these filters are usually very expensive. One problem with this filter is that the "sump" can overflow and possibly drain the tank if the pump fails. You can also install the heater within the "sump" instead of having it inside the tank .
You will need a heater to keep your tank temperature constant. There are 2 types of heater, submersible, and ones that hang outside the tank. I personally prefer the submersible ones because they can be placed anywhere inside the tank, but these models are a bit more expensive than the hang over types.
You'll need a thermometer to keep track of the temperature, some thermometers stick on the outside of the glass, and some are attached by a suction cup on the inside of the aquarium. Personally I prefer the suction cup ones, because you can remove it and use it in another tank. The stick on types are permanent. However, some heaters come with a built in thermometer, so you may not need to buy one.
These are good for keeping the water cirucalating. Most Piranhas like to swim in the current of the power head, but you don't really require one if your filter provides enough circulation. These can also be fitted with a "pre-filter" to help with mechanical filtration.
Air Pumps are used to keep the water oxygenated, or run certain filtes, but these are usually not neccasary if your filter creates enough surface movement on its own.
Selecting a healthy Piranha:
Before debating to purchase a Piranha first take into consideration if you can properly care for the fish and meet its requirements. If you can't, or are not willing to provide the proper conditions for the fish, then you should reconsider your choice in buying a Piranha. Sadly, Piranha are one of the most abused of aquarium fish, because people buy them on impulse without even knowing how to care for the fish. So please, do not buy any unless you are willing to provide proper care for them.
Before purchasing Piranha(s) first observe them. Piranhas are usually kept under crowded conditions in petstores and it can sometimes be difficult to find a healthy specimen. Here is what to look for and avoid:
What to look for:
The Piranha should be very active and alert of its surroundings, no severe body dammage (minor nipped fins, are ok), both eyes intact bright and clear, should readily eat, respiration should be normal, no signs of external parasites.
What to avoid:
Lethargic, sluggish Piranha, not willing to eat, severe body dammage (bites, chunks of flesh missing), cloudy eyes, missing eye, labored respiration (could be a sign of gill problems), parasites on the body.
Routine aquarium maintenance is essential for the health of your Piranha(s). This usually only takes about 10minutes of your time every week.
You should change the aquarium water at least 20% every week. There are some products that make this very easy. Their called "Pythons", they hook up to your kitchen/bathroom sink, and are a big help when draining and re-filling tank water.
Some filters do not need as much cleaning as others, I'll include here the types of filter and how often they should be cleaned. First, any filter media such as the sponge, or any biological media should be soaked and cleaned in a small container of water taken from the aquarium. If you wash them under tap water you may dammage or kill the bacterial colonies.
Canister Filters should be cleaned every 4 to 6 weeks. Power Filters should be cleaned about every 2 weeks. Sponge Filters, Under Gravel Filters, and Inside Box Filters should be cleaned every week.
Weekly, you will need to 'vacum' the gravel to remove all the left over food and fish waste. If you have the "Python" (water changing device) you can vacum the gravel while changing the water. Or you can purchase a gravel vacuming device that hooks up to an air pump.
Whenever neccasary you can scrub the sides of the tank to get rid of algae. There are a variety of scrubbing devices made for this purpose. The best kinds would be the magnetic ones, because you can clean the sides without sticking your hands into the tank. Personally, I only scrub algae off the front of the tank (so I can still see the fish), and let algae grow on the other sides. Algae is just harmless primitive plant life.