My friend has an oversize 2.5foot tank and wants to know if it s big enough to keep 1 oscar in. It hold 165 litres.

point blank No. oscars grow very fast, at a rate of say 1.5 inch a month. so in 6 months your mates fish would be 6" or more in 1 yr it could be half the tank size. a tank of 55 imperial gallons is whats needed for just one oscar. along with a very powerful filter of twice the tank litre. if you try it any other way the fish will become sick, sufffer,and then die and maintance will take over your life. My advise is try guppies or something much smaller and build knowlegde on fish keeping.

Yeah never ever put a Oscar in a small tank like that especially if its around 20US gallons that will never be enough room for an Oscar unless it's a baby Oscar but even then so it will out grow that tank in a hurry give it less than a half month just about and that Oscar will be bigger and hardly move around in that tank...My smaller Oscar is in a 10US gallon tank for quarentine for healing thats all