Wow.... you did almost everything from scratch...
Good job, however you made the one biggest mistake in our hobby with water.
That is the mixing Aluminum and Copper in the same wetted loop.
When i say wetted, i mean water contact.
In the presence of a electrolyte.. ie current from your computer, you can initiate galvanic corrosion. This can be very bad in a closed system.
The easiest fix is to stick with metals close to each other in the galvanic table.
Ie. Brass + copper... Nickle + copper.... Some SS steel will also rust, so make sure u get good steel.
We dont like using chemicals to slow gavlanic corrosion.
Usually the chemical needed is either massive quanitites of Ethly Glycol, or water wetter.
Both very bad because they lower the holding capacity of water by adding viscosity to it.
Seeing how your an avid modder, have you tried to aquire an old heatercore from a bonniville?
We call them bonnie heatercores, there made out of copper, and should take care of the alu rad i saw.