I use a cheap pump (Whale brand) sold for water supplies in caravans and boats. In theory, it's not rated for continuous operation but I run it on 5V instead of 12V (but with 12V available for filling the pipework when needed) and it runs for hours on end. Virtually silent running like that - I have to look for the odd bubble in the pipe to check that it's running. Never had any problems with overheating. I've been using it for years now. Others feel that more cooling is needed and go for PC water-cooling fans and radiators (which reduces pipe runs) or more powerful pumps but my experience has been that the cooling needed is often over-estimated. Maybe I just don't work my 2.2KW spindle hard enough...