How much should i change the water?

how much should i change the water in my piranha tank like days and how much water and what do i do iin the winter when iu cant change the water?
Answers:
You should do water changes once or twice a month with a siphon hose removing approximately 25% of the water. This is of course assuming the tank is the right size.

Knife fish usually get from 1 to 3 feet depending on what type it is. For 3 at least a 75 gallon probably.

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I think you have to change the water every week. 50% water change sounds right. Just try to keep on schedule.

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First off..what type of piranha, how many and what size tank? Why can't you change the water in the winter?
Depending on the type of knife fish, an adult can be 12 - 24 inches long and will require 100 gallon tank. They cant' be kept together. Even if they were the same species of knifefish.

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Does your tank have a filter? If so, you can get away with changing the water less regularly.
If not, change 10% of the water volume every week with aged water.
The easiest way to do this is get a 2L soft drink bottle (or more if required) and fill it with water. Leave the bottle on your windowsill with the cap removed for a week. This ages the water and removed the harmful fluoride, chlorine and whatever else may be in your water.
If you are lucky enough to have a rainwater tank, use this water in your fish tank - you don't have to age it.

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I can't tell you how often to change the water, since I am quite negligent when it comes to that. Probably once a month is good.. I'd take a couple of gallons. As far as the changing water in the winter goes, I bought a vacuum hose that you connect to your sink faucet (unscrew the aerator and screw in the hose). It came with like 25 feet of hose (or sometihng like that) and you can control the flow of water into the vacuum. Change water all year long. Just make sure your fish arent hungry when you change the water, they may nibble on your hand.

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Siphon a quarter to one third of the water and replace about once a week or so. If you do that it should never have to be completely changed which often causes shock and can kill your fish.

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For piranhas, at least 30% weekly. Be sure they don't bite off your hands first!

~ZTM

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Changing water once or twice a month is not going to be enough. You should do a 25% to 30% water change once a week and maybe twice a week depending on the stocking level of your tank. Use a gravel vacuum and be careful to not startle the Piranhas. Just use tap water and use conditioner to remove the chlorine from the water.