Mike, change the water...
You are suffering an endless problem and that's why we cool with fresh rather than raw (salt) water. When I was installing a Nova Kool 12V unit on our last boat (in Annapolis, where we lived aboard), I plumbed the cooling line off of the main tank line using a 'T'. After the fresh water cycled thru the condenser/compressor unit, I 'T'd it back into the tank at the vent line. When installing our current system (an Isotherm ASU 12V holding plate unit, and a real gem), I added a fine filter to protect the Shurflo water pump but it rarely picks up anything as we're careful about filling our tanks. We did the plumbing about the same, except with 3 water tanks, I 'T'd off one I was comfortable leaving for last to use (it's in the center of the boat). We've used this approach on two boats now, over a combined 10 years with 6+ of them living aboard, and it's never presented a problem. It really works well. Many initially dislike this idea, believing the water in the tank will heat up as it cools/recools the condenser/compressor. It really can't, as the tank water will find it hard to get much above the temp of the water in which the boat is floating. Almost all tanks, one way or another, are heavily influenced by the outside medium. An engineer I was helping this project with in Trinidad didn't believe this claim and ran a week-long experiment. He then changed over to fresh water cooling for his separate freezer and icebox systems; the most temp increase he saw (using a 5 gallon tank) was 5 degrees. Our Nova Kool unit could also run as solely air cooled, which allowed me to verify the efficiency of water over air cooling, but our current system MUST be water cooled and so we take care when using that last tank. If your boat lacks multiple tanks and/or you push the envelope on using up all your water, I'd recommend a tank meter...and perhaps carrying a jug or two of spare water with you. Jack
