I know man but inside its the same as any cooler system has copper in it..
Listen i have a big protein skimmer which uses a lot of air, am sure if i can cooling that air i will cool the hole tank,any idea!
Actually most modern day car radiators use aluminum radiators with plastic or aluminum tanks because they can build them thinner and are just as efficient and stronger than the older thicker copper radiators on older cars. Both of my cars one is a 1999 and the other is a 2001 have aluminum radiators but the only way I would even think about trying this is if it was brand new and you could flush it out for a few days with some vinegar/water pumping through it! Never know how they are manufactured and what stuff is dumped and run through them though. It would be a chance but I could see how it would probably work pretty good at dropping the temp down. If you decide to attempt this document your findings on here as it might be a way for others to cool tanks as well.
Have you thought of how the saltwater will eventually react with the radiator? I'm not sure about aluminum but saltwater and metal generally don't mix. Just a thought. Maybe someone here will have experience with saltwater and aluminum.
I wouldn't run tank water through it no matter what...use an intermediate fluid (water or alcohol or ???) in the radiator and some food-grade/tank-safe tubing to run it through your tank water in a self-contained loop.