PH during a fishless cycle.

I am by no means an expert, but my guess is that the nitrification process is eating up your buffering capacity. Can you test your kH (both tap and tank)?
If it is low, you will need something to buffer it up. During a fishless cycle you add so much ammonia and don't do water changes that this might be the reason...

Stefanie has the right idea. Nitrification uses up alkalinity/KH as it is an acid-producing process. When your pH drops below about 6.5, you are in danger of slowing down the growth of the bacteria you are trying to cultivate. Add very small amounts of sodium bicarbonate, baking soda (from the grocery store) and keep testing the pH to get it back up to at least pH 7 or 7.5 and keep it there during cycling.

HTH

I don't see any reason not to accomplish the same thing by water changes. In an established tank, the nitrification process is trying to pull down the pH, too, but the effect is contained ultimately by water changes (new water has higher pH and new buffering). Water changes shouldn't hurt the cycling of the tank.

Any debate? Anything I'm missing?

I think that if your tap water is already low in kH that even water changes might not be enough to buffer the pH reliably. During a fishless cycle you add more ammonia daily than a normal fish load would produce, so I think that especially during cycling it is important to have enough kH.
Maybe someone else has more input?

I think that if your tap water is already low in kH that even water changes might not be enough to buffer the pH reliably. During a fishless cycle you add more ammonia daily than a normal fish load would produce, so I think that especially during cycling it is important to have enough kH.

Click to expand...

Hit the nail on the head again Stefanie . If your tap is high Kh then yes water changes would take care of it. but 7.7 ph doesn't really indicate a lot of Kh at atmospheric co2 levels. So water changes will be less helpfull than they might with higher Kh tap water. Additionally, if Tap Kh was high to begin with, you problably wouldn't be seeing the ph drop you have.

Water changes are generally never a bad idea, but one of the distinct advantages of fishless cycling is the elimination of the constant water change need during cycling. Baking soda is much easier, and will buffer the tank quickly during the fishless cycle.

As Stefanie pointed out, once the fish are in the tank the bio-activity will be greatly reduced and thus the Kh consumption won't be nearly as high. after that point water changes will probably suffice.

http://www.aquariacentral.com/forums/showthread.php?t=57791
Dave