i had 2 pirahna in a 20 gallon before...6inch long..the aquarium i built was like 16x40x16 or something like that.and they always were happy with the litle curent i had...yes the ideal tank would be larger...but i'm not rich,and never built a big aquarium as i'm not to sure how to glue big ones.i am a glassman....
i say 40 because i saw a good deal in a store,40g aquarium with canopy/neons,filter and stand....for 350$
i could build myself one but at this price....
i was thinking about 4 piranhas or 2 snakehead
the kind they have now is like orange and 3 inch long.if i remember they should get 2 feet long.but if i go with snakehead,i will for sure get a bigger tank as they grow.