Our comet definitely has some stunting issues (was kept in a 15 litre tank with two other fish, then a 90 litre before we found out about goldfish). He still hit ten inches no problem. I hate it when petshops sell them - our local does a deal for members where if you buy a 20 litres tank, you get TWO free comets. Grr.
He's going in a pond in a few weeks with another rescue. :)