Quiet One 4000
****
Pro: Inexpensive, well made, powerful, efficient, and quiet
Con: The connector for the hose is a straight thread sealed by an "O" ring.
Comments: I would (and probably will) buy one again.
Rena XP3
****
Pro: Fairly decent media capacity.
Con: Weak pump, messy to clean, clamps feel cheap.
Comments: I'm glad I only paid $50 for it, but I question if it's even worth that.
AquaClear 110
****
Pro: Easy to maintain, low bypass, freedom of media selection, large media area), easy to replace parts
Con: Impellers can jam easily with sand.
Comments: I love the AC filters. With a sponge over the intake they are great in almost any tank.
Little Giant 4-MDQX-SC
****
Pro: Reliable, low heat transfer, built very well.
Con: Has a slight hum.
Comments: Great for setups where reliability is a must.