How long had the tank been set up when you saw the sudden spike in your ammonia? Are all of your fish accounted for? Could one have died? Did you have any shrimp or a starfish or something else in the tank that could have died?
With a 125g tank and high ammonia doing a 10g water change isn't going to help a lot. You really need to look at 30g or more to have an impact. Regardless of whether your cycle is complete or not you need to get those ammonia levels down.
Walmart carries 32g trash cans for about $12 that are perfect for mixing SW
in. I have 2 that I use for this purpose. See if you can do a larger PWC
to have a greater impact on the levels.