The U.S. Public Health Service recommends that the total solids of portable water be limited to 500 PPM, but if such water is not available a total solids content of up to 1000 ppm may be permitted. While this varies with the lake, they usually have 250 ppm or less. Suspended Solids are the things floating in the water. This can be algae or dirt. All lake water will have some of them, but the lower the better.
Algae blooms can cause the suspended solids to increase quickly.
Secchi disk reading is a measure of the clarity of the water. It is measured in feet. Generally, the higher the number the better. Secchi disk readings are usually better in the spring and fall but lower in the summer due to algae growth and boats stirring up the water. In pollution free mountain lakes, the readings can be as high as 50 ft.