First of all, some tank info would help greatly. What size tank, how long has it been running, type of filtration, tank inhabitants, temp, and the results of any water tests. Most lfs will do water tests for you if you don't have the test kit at home. If you called your doctor and told him/her "I feel crummy" it wouldn't give them much to go by in diagnosing your ailment. The same holds true for aquatic ailments.
If it is more of a milky color, it's a bacterial bloom, usually associated with newer tanks, overfeeding, too much cleaning of filter media or substrate, or a "missing" fish hidden away somewhere decaying. These usually run their course and solve themselves, if you are missing a fish, it's best to look for them & get them out of there.
If it's more of a greenish color, it's am algae bloom, caused by too much light, infrequent water changes, or overfeeding. Cut back on lights & feeding, and try a daily water change of 10% for a week.
The net bag they are referring to is similar to the net bags thay onions or potatoes come in. I've found that one of the wife's old nylons works just as good. Anything clean that would hold the substance in and let water flow through, as well as fit in your filter should work fine.
Do not remove all the media out of your filter, just part of it if you plan on using this clean water, as it will cause your tank to re-cycle, assuming it is cycled.
Tolak