I have a GT in my 180g that I bought in February. He was about 2.5", he's 4" now. Mine is the first to eat, the most people-friendly fish in the tank, and is generally a dick to all the other fish even though he's the smallest. He's not stressed and gets plenty to eat, yet he hardly grows at all. Just the nature of the beast, I guess. I would imagine that genetics, as mentioned by others is the #1 factor that determines growth.
A long time ago I had a 45 gallon tank and knew nothing about fish, ( I bought it from someone else and it was my first tank). It came with a fully grown Oscar, five 7+" GTs, five 10" common plecos, and a red-tail shark. I have no idea how they all thrived, but they did for years and years. I think when the largest GT finally died 5 or 6 years later, he was about 10". I haven't seen a larger one in person since then. Definitely slow growers even when you have one with "quick growth" in their genetics.
A long time ago I had a 45 gallon tank and knew nothing about fish, ( I bought it from someone else and it was my first tank). It came with a fully grown Oscar, five 7+" GTs, five 10" common plecos, and a red-tail shark. I have no idea how they all thrived, but they did for years and years. I think when the largest GT finally died 5 or 6 years later, he was about 10". I haven't seen a larger one in person since then. Definitely slow growers even when you have one with "quick growth" in their genetics.
