We carry a full line of Kasco Products.  You can purchase the Robust Aire series diffusers up to RA3 here on our website.  Its recommended that you contact us to purchase the RA4 up to RA6.  We want you to be completely satisfied with your purchase and experience.  We will give you a free customized aerial layout of your system.  So you are getting exactly what you need to improve the quality of your lake.  You can use the contact form below.  We make it priority to give you a call back as soon as possible.  Thank you..