arrrgh!!! what happened???

I think it was probably the rocks. If you put a couple drops of vinegar on the rock and it fizzes, it'll probably raise your pH. You can do this with any acid. What you're doing is checking for carbonate content which is a base.

If you remove the rocks, I believe the buffer they added would remain in the water until removed by water changes. My first thought would be to remove the rocks and do a series of small water changes over a few days to slowly bring the pH back down, but someone else here may be able to give you a more authoritative answer.

I use holy rock (limestone) in my tanks, and they will raise your ph somewhat, just make sure for the first 2 weeks after adding it to do daily 10% water changes if not 20% every other day and that kept my ph in check. ofcourse my tap water ph is a hair under 7, this wouldnt work if your tap water ph is high. Also I was doing some reading, its not usually the ph shifts that kill/stress fish(as long as its not HUGE changes), its the ph shifts on tanks with low kh, because after hearing someone else mention it, I tested the ph of this BIG pond behind my moms house, and it will fluctuate around 2 points from morning to night, and the fish in there dont just up and die...FWIW.

Unless you added a lot of calcareous rocks, I doubt your pH would jump that high, especially with water that was starting at 7.4. Have you tested a sample of your tap water after it has sat out overnight in a shallow bowl? It is not uncommon for tapwater to contain dissolved gases that drive down pH (e.g., carbon dioxide). Allowing your tap water to age before testing for pH will give you a more accurate reading.

HTH,
Jim

The rocks as said could make a difference, but It wouldn't seem to me to be that fast. IF you tap water is 7.4 after gasing off what is the KH at that point.
I would definately remove the rocks until further investigation, and do the series of water changes to get things back into control slowly, and then proceed once the tank is back to whatever was normal. I imagine you tap KH is much lower than 5 with a ph of 7.4 after gas off. Therefore the rocks may be the issue. As said Ph swings arent the real big deal. however quick changes in TDS levels and or kh can be a big problem for fish. The flip side of that is that if the rocks changed the kh they wouldn't do it quickly and the fish should adjust.

The other question is where did the rocks come from and what are they. many rocks contain traces of heavy metals that can do some weird things. I am a firm beleiver in going out and picking up free rocks, but there are good and bad sides to doing so.
dave