[StephenAK]

Hi guys, I'm a chemistry noob, but I think that will go without saying...

OK, I'd really appreciate some help with this problem I'm having with EN plated copper. I'm working on a product for agricultural hydroponics, a nutrient temperature control system, and I'm working with copper heat exchangers (blocks) that the nutrients flow through for temperature control.

I soaked one of the blocks in a typical hydroponic mixture which is distilled h2o and nutrient additive. The solution is adjusted to 5.5pH. It took less than 24hrs for the tarnish to cover the entire submerged area and within a few days some funky new colors of tarnish were showing up.

The EN accumulated some kind of black/brown and in some spots almost copper"ish" tarnish, which was easily removed with a scotchbrite pad and water. There is a faint amount of this tarnish residue left, but I didn't vigorously scrub either.
 
I did a lot of research on this via Google and the best thing I can come up with is that the phosphoric acid in the hydroponic nutrients is causing the tarnish/corrosion/oxidation (whatever it is...). Could this be the case?
Here is a list of the "guaranteed analysis"

5% Phosphate P2O5
4% Soluble Potash K2O
1.5% Magnesium (H2O soluble)
1% Sulfur

Derived From:
magnesium carbonate, magnesium phosphate, magnesium sulfate, phosphoric acid, potassium carbonate, potassium phosphate, potassium sulfate.

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Stainless Steel works great but because of it's poor thermal conductivity I can't use it... Would gold plating the copper work better? Any other suggestions to protect the base metal?

Thanks in advance for the *delete me*