I was informed from the store where I got my beer kit that after secondary fermentation was complete that I should top off the carboy so no bacteria forms in the head space. I was told to use tap or spring water to fill the carboy from 1-3 inches below the stopper. Does this sound right?

You can use water you do not want to use a ton of water. In winemaking many people add class marbles to the tank to bring the levels up.

You can add water if you want too. It will "dilute" your beer though. If you're infected with bacteria, topping off will only reduce the amount of surface area. If someone suggested that to reduce the amount of CO2, I'd say, don't worry about it at all.

Thanks for the replies. I did not think it was a problem without adding the water but since I removed the bung to take a final gravity reading, I thought it might have let in bacteria. This is my first time brewing and I brewed English Brown Ale and used the glass carboy for the secondary fermentation. I'm sure I will have many more question in the future and it's nice to have a place to go for answers. Thanks again!

Topping off really isn't about a concern with respect to contamination.

People some times suggest doing so to reduce that amount of beer in contact with the air thus possibly reducing the changes of oxidation. This is for the case of using a carboy. Because of the container's shape more fluid results in less surface area in contact with the air. In the case of a bucket the surface area is essentially unchanged though the volume of fluid is changed.

However, in both cases the need to reduce the contact area is quite minimal. The CO2 produced by yeast activity is denser than air. Therefore, the beer/wort is protected from exposure to the ambient air by a blanket of CO2.

Okay, I used to top off my carboy with water not because of any concern with respect to oxidation or contamination but rather to maximize my beer production. I had five gallon carboys but I usually only put in about 3.5 to 4 gallons of wort into the primary fermenter. This allowed for sufficient headspace so that unless the fermentation was extremely vigorous I would not have wort spewing out through the airlock. I designed my recipes based upon five gallons so that the dilution of topping off would not effect the end result. I generally increased the hopping rate slightly (i.e. ~5-10%) to account for the lower utilization associated with concentrated wort.

Have primary fermentation was done I would transfer the beer to a secondary fermenter and top it off with water to the full five gallons.

If you decide to top off in the secondary there is something one should be concerned with, however. At this stage the yeast should not be exposed to oxygen. The water used to top off should be de-aerated first by boiling and cooling. Not only does this reduce the risk of oxidation but also helps kill off any bacteria or wild yeast that may have been initially present in the water. When transferring top off water try not to splash so air/oxygen is not introduced.

I hope this helps a bit.

- Scott

I've come across a few forums where brewers are talking about topping off in the secondary. To me it doesn't seem necessary at all, or worth the risk of contamination by screwing around with what's been successfully fermented. Pros don't do it, why would we?

Topping off wine in the secondary is necessary because the fermentation process has been halted by the addition of sulfites, so reducing the airspace will reduce oxidation. Beer is not wine, and the process is different. It's not worth the risk at all. I have never topped off any of my brews. As brewers I think we should be concerned with adjusting our mash and boil volumes to end up with the correct volume for fermentation in the first place, not adjusting the level at the end with plain water.

Check out this recent post on the subject...
http://cryptobrewology.com/nips/topoff

Just MHO