If you have access to gulf water, here's what I would do: I'd circulate small amounts in and out during hours. Enough to get rid of the contaminants, but not enough to drastically affect the temperature, which should be regulated pretty much by the surrounding atmosphere. In other words, you don't want really warm gulf water in the summer, or too cold water in the winter.

One of the first tanks I kept (many, many moons ago...), I kept by dragging a bucket or 2 of ocean water down the street every day!