If the pump really was causing EMI a sheet of aluminium foil around the pump and touched to the case would be sufficient, but i highly doubt that is the cause.
Just to rule stuff out, try this....
get a long mains exstension lead and plug your monitor into it from another part of the house. if that stops it, your wiring in that part of the house is on the fritz. i got my dad to rewire my whole room just so my computers would stop crashing randomly due to power fluctuations (before PFC Supplies), otherwise i had to have everything plugged into a socket 2 floors below.
Do the same with the pump if mains powered, or use a seperate PSU to power it if 12v. (plug it all in, then use a peice of wire to short out the green and a black wire on the ATX Molex) that will rule out any power fluctuations in the PC itself.
I have had a Eheim 1060 sat in a computer, right next to a monitor with no problems before, but i have also had the same setup turn a monitor into a kaleidoscope. no two setups are identical
