Your "pilot" test says a lot. If the color is eaily removed with carbon at a high flow rate, you may be able to simply replace the media in your tank with GAC. It is much cheaper, no regenerant, and it also removes many chemicals that may or may not be present in the lake water. Do you have a lot of power boats in the lake? I sell a lot of tannin removal equipment, but I am not a big fan of it. The problems it can create sometimes outweigh the benefits. Try this test, run water though the carbon filter at a high speed, as fast as the filter will handle. Does it still remove the color? Do you have the model number of the filter? The color can be from many sources, and some may be removed by carbon, tannin resin, or even just the micron rating of the filter. It will cost almost nothing to do the next test. buy a 5 micron or 1 micron sediment filter cartridge and put it in the filter housing in place of the carbon. See if that also clears it up. Please post the test results when you get them and we will come up with a great and reasonble priced solution for you.