a question

hello to everyone!I have two reds (nearly 3inchs each one)in a 33 gallon tank about 4 months now with an biological internal filter (recently i changed its original powerhead with a stronger 600 litres per hour)and a bottom filter powered with another powerhead (600litres per hour) all from hydor.I know that 33 gallons are few for two reds but how long can i keep them in this tank?The man who sold them to me from the pet shop told me for life!because fish grow according the space you give them.Can this happen?And if yes which will it be their final size?thanks