How to shorten E.Quadricostatus


I've got tired of my tall growing E.Quadricostatus. They are about 
15-20cm tall, some taller than my E.Amazonicus.  I need it to grow less 
than 10cm. I saw the same thing at the fish shop and their runners are 
less than 5cm tall. It must be the CO2 as the shop do not inject any 
CO2. Someone please advise me how make it grow shorter. Don't tell me to 
use E.Tennellus because the shop are not selling them over here. Thanks.