Becasue the eheim 2213 only pumps 440L an hour so thats 880L having two and on a 250L tank, so there is only 4x water turnover (if they are still pumping at there max, im not sure if they are). Plus I am planing on getting some smaller tanks for breeding (thinking cory's) and I can use the eheims on these. At lest thats what I was thinking :-?