You know they are going to out grow that tank right? I'm guessing your putting them in a pond when they get bigger. I use the 40g for the first one and 20g for ever other comet. If you want to keep all of them in the same tank you are going to need a 260g tank.
Absolutely normal. I have two that are 12 inches long at 2 years old. I got them at less than an inch long. Yours will outgrow that tank in no time. Not to mention the larger they grow, the more waste they will excrete. I'm amazed that you've been able to keep up with with the water changes right now.
i know they will outgrow it but i needed to cycle a tank, but soon since it is cycled i am going to put them in my pond (300 galon), there should be room.