Gather some eelgrass at low tide by scooping up the mud around its rhizomes by hand. The mud holds together best when it is cold, so early spring is a good time to do this. Collect these in a tub or bucket. Also collect enough mud to cover the bottom of the tank about 2-3 inches deep. For short term tanks (1-2 weeks), gravel can be used so that the water will be clearer. Partially fill 4 to 5 five-gallon buckets of sea water to the level that you can carry them. Lid them for transport in your vehicle.
If the tank will be kept outdoors, put the tank on a stand on an even, solid surface in a shaded spot, or use one of our stands which has a plastic roof. Add the mud you collected (or gravel) to your tank. Add the eelgrass plants by placing the rhizomes in the sediment and covering them as best you can.
Slowly add the sea water by ladling it with a smaller container and pouring the water against the side of the tank to prevent a heavy stream from disrupting the sediments and washing the plants out of the mud. We have also found that holding an edge of the small container horizontally just under the surface of the water in the middle of the tank and sliding it slowly to the side is a good way of emptying the water without it stirring up sediment. ‘
Add an oxygen aerator to the tank if it is going to be kept in the classroom. (Though the eelgrass produces dissolved oxygen, the bacteria in the closed system of the tank tend to take up more oxygen than the eelgrass can produce.) Evaporation will make the salinity go up. If you want to add freshwater to bring down the salinity, be sure to treat it first with an aquarium water treatment solution to remove chlorine and chloramines. Read and follow all package directions carefully.
Replacing water periodically with fresh sea water will extend tank life!