I thought about buying a piece of titanium tubing (can buy a 3' length
for $40 or so) and bending it into a coil.
Then pump water through it and blow a fan across the coil.

hi.
Two points.
1, titanium tubing is a real PITA to bent, even if you get the softiest
grade.
2, the fact that the fan cools your sump is because of accelerated
evaporation. The evaporative cooling is what take the heat out of the tank.
With the fan blowing on a titanium coil with warm tank water, you might try
to cool the tank by blowing air on the front of the tank. It won't work
since it does not promote evaporation. You need to put the coil in some
cold water bath or something.
bye -ck-