Lilly's on a 20l? Lilly's are nice but they get dirty sooo quick, I have a rimless 120p and never even considered Lilly's just because they are a pain to keep clean IMO.
I suggested a 2215 on the fact that I have a 2217 and a 2026 on my 120p, the 2026 QD broke on me and I only used the 2217 for a week or so but I felt the 2217 handled the tank fine. I can't even imagine one on a 20l. Should be a whirlpool but looking forward to see how you like it, it's prolly my fav filter ever.
Good luck with it!
EHEIM Pimp Club Member #250