I have a 75 gallon tub
A single 2-3" koi and don't feed it so it won't outgrow the tub too fast.
PLEASE don't put a koi in a 75 gallon tub....it WILL get too big if you feed it
or not. I think 75 gallons would be too small to support a koi without food.
How about a coule of fancy goldfish about 2 or 3 of them ......Jerri
http://www.fringeweb.com/Ponds/JerrisPond