In our zone, zone 5, we don't recommend submerging marginal plants. The ones we use around the ponds are zone hardy and don't need to be submerged for winterization. I would assume that the brown water is from the soil from the marginal plants.

Since you didn't say what kind of de icer you have, my response is for both types of pond de icers. Both floating and submersible pond de icers are designed to turn on when the temperature of the water falls to 32 degrees (freezing level) and to shut off when the water temperature reaches 45 degrees.

How large is your pond? Use one de icer for a 600 gallon pond. If your pond is larger than 600 gallons you will want to use two de icers tied together. It should be placed as close to the liner as possible as the heat will reflect back and in the shallowest part of the pond.

I hope this helps! Take a look at my profile pictures for the Koi Pond that we built this past summer.