Clown knife fish tank

What are the dimensions of your tank? He should be able to get a good 6 months out of it if hes only 3". Although not the most active during the day, knifefish need lots of open swim space at night when they are most active.

Mine grew 10 inches in about 3 months...it all depends on how well he eats for you and how much you feed him. Also depends on what his tankmates will be in the 200gal.

used to love watching my CK swimming around at night, venturing to the surface to grab a big gulp of air. awesome fish, if you have the 200 gallon tank than why not set it up and give the CK even more room to thrive. he would certainly grow quicker in the bigger tank given you feed him sufficiently and the water quality is maintained. but if you plan on stretching out the life of the 72 gallon i would have to say between 6-8 months before you should need to move him to something bigger.