Does anyone know if the 400 is a good filter for a 55?
It's a good filter. I prefer the AC110 now made by Fluval ]
400's are good filters, BUT over time the bearings that the bio wheels ride on wear, and ther wheels stop turning. If you get one make sure you can order extra bearings when needed. I got rid of all my emperors, and went with 110's, no cartridges to buy