I have an fx6 on a 120 (4ftx4ftx2ft); and it won't stop knocking. Fluval said to replace the seals which i did - still knocks (kicking out air i guess); anyway the thing is 4 years old and i rather spend my time on something else - so is there a good alternative? The tank currently has 5 krobia and 14 angels with a few 5 inch pleco (4?). I'll be giving away 1/2 the angels soon - so i just want something that will do the job quietly. A bit of current is fine.