Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Test the water with all the tests you have. Some of the chemicals might be reflected in the material you choose (such as high GH or KH). Some of the materials you test will be rinsed out of the substrate before you use it, so are nothing to worry about (Nitrogens).
Take a scoop of the material you think is good, and add some RO or DI water, and test that every few days for a week. If the GH, KH, pH or TDS increases over a week, then it will do the same in your tank. (This may be good or bad, depending on what you want)
Take a glass jar with you the next time you go to the creek. Scoop up some 'silt' and add water. Shake the jar really well and time how fast stuff falls out of the water.
30 seconds = Sand. Good.
2 minutes = Silt. Pretty good, as long as there is not too much.
over 2 minutes, and if the water stays cloudy = clay. Not so good. A little is OK, it helps hold nutrients in the substrate, but too much simply leads to anaerobic pockets in the tank.
Test the materials at several areas along the creek. Some areas may be more sandy than others. Dig deeper and see if the material is the same a bit deeper. Or else plan on just skimming the top couple of inches to get aquarium substrate. Do not worry about animal poop in the water, this does not cling to the substrate too well, especially a predominantly sandy one. If there are any mine tailings in the area run, do not walk away. The heavy metal contamination is nothing you want to deal with.
Live fish and other things in the water suggests that there may be fish diseases and parasites that you do not want to introduce to your tank.
Bake, boil, or air dry exposed to the sun to kill these.