Around 75 gal preferably more when small. School of 5+ (the more the better as the hunt in large groups in the wild and it's the same in a tank) 100 - 120 gal when they get bigger. If you were to get more than 5 then a bigger tank will be needed. They can get to a chunky 12 inches each and are very messy eaters so you would need a filter capably of turning over at least 3-4 times the tanks water volume and preform a 30-40% water change weekly with a vacuum/syphon. You can get them Trimar, they are based in Cornwall but also do a mailing service to mainland uk for £12.95 (Trimar.co.uk).. The piranhas themselves go for around £6 each on there at a very small size. Full stock list and prices are in the website. I got my snakeheads from there, excellent service. Ordered 11am got them 10 am the next day. You will need to call them to place an order and call before 12pm for a next day delivery and they deliver between 8-12pm Tuesday through to Friday