Geez I saw that thread title and was like, Oh noes! lol

Well, the bacteria you speak of probably aren't going to survive in a hydro environment, between the different chemical makeup of the res water and teh beneficials that a lot of us are using (haha yay 'good bugs')... but precipitates in hydro ARE common if you're using cheap or incompatible nutes, nutes intended for soil, or have really hard tap water, or are using the wrong shit for pH up/down (pool chems = baaaaaad!!!!!).
Oxygenating the res is important especially if you run beneficials in it to control fungus and bacteria. The pump instead of airstone works fine in NFT and ebb n flow settings, but in DWC you still need a super duper airstone.

Now, get some more damn sleep already!