what happens if I run a cooper pipe into an aluminum tank. If I keep the two metals separate, will I still get corrosion? The pipe would be submerged in water. Obviously the tank would be filled with water as well. But the level would change on a daily basis. The temperature would remain a pretty steady 90°F.

Baluncore said:

Welcome to PF.

The different metals are the plates of an electrochemical cell.
The water is the electrolyte.

If the metals are connected, there will be rapid corrosion, as the connection represents a short circuit across the cell. If you can separate the metals with a long length of plastic pipe, or the water was very pure, then the cell may not discharge, and the metals will only dissolve in the water slowly.

You can work out the voltage of the cell using electrode potentials.
https://en.wikipedia.org/wiki/Standard_electrode_potential_(data_page)

Now there is something I hadn't thought of. It would be cool if I could harvest the electricity. I doubt it would be worth disolving the plumbing in my heater.

I'll not say 'DO NOT !!' but, IMHO, putting such electro-dissimilar metals into proximity is a very poor idea.

IIRC, there are stiff-ish 'potable' grade polymer laminate pipe types which would side-step issues..