FWIW...
I just pointed out in another forum that the Rio 1100 would be ideal to run in series:
Max head: 6 feet
price: $19.99
price @ qty6: $17.99
Max flow: 382 gph
(from:
www.greystonecreations.com btw you might still get the ProCooling discount)
Putting 6 in series would give a total head in the order of 36 feet (10.97 meters, or 15.6 psi).
The problems are:
1-the pump housing may need reinforcement, as the amount of pressure may cause leaks.
2-this pump may or may not last long, in-line. Rios are very popular, but the bigger ones can get fairly warm. They are designed to be run submerged.