Lance is correct. They have improved the ECCO line, but the line doesn't replace the Classics. The Classics are still available.
The older ECCO's has a somewhat different filter design that the new ones. I do not know exactly what all has changed. I have an ECCO 2236 and it is posted that we make sure that we are getting the correct pads.
There was even an older ECCO line several years ago. Their last number was an odd numbers whereas the newer ECCO's last number's are an even number. It could of been 2231, 2233 and 2235 vs 2232, 2234 and 2236.
New ECCO Pro Line (Easy 35, Easy 60, Easy 80): http://www.eheim.com/files/image/fil..._userguide.pdf
Older 2232, 2234 and 2236: https://www.eheimparts.com/client/homepage.aspx