I struggled with the same problem last year. How to test for dissolved oxygen at various depths in my pond, except my pond is about 15' deep at the deepest point. I bought a very small battery powered submersible pump. Connected approximately 20 foot of small diameter plastic tubing and 2 conductor 18 or 20 gauge wire to the pump. I simply lower the pump to the desired depth, connect the battery terminals to the pump wiring, let the pump run a short while until I am sure that all of the water output is from the required depth and fill a plastic bottle with the desired water sample. I then take the water bottles that were taken at various depths from 1 to 15 feet back to my cabin for analysis. The test results have so far been very informative. I would estimate the total cost of my sampling system to be no more than $50.