Okay, well, here I go. Firstly, any leftover food is too much. Never feed fish more than they can eat. I actually feed them only what they can devour within about 30 seconds twice per day and I also fast them one day a week. Some people only feed every other day. With the deep gravel you mention, I am just imagining all of this accumulated uneaten food decomposing and producing your ammonia problem. Start cutting way back on feeding. Uneaten food won't lead to a problem right away, but after time as it begins to, well, rot; it will pollute your tank. Maybe this explain the month of them being fine. Also, it never hurts to call your water company to see if they've added anything new to the water.
As for the pH, wow. I don't have that problem. My water is hard and alkaline. You might get some advice under a separate post for that, but I cannot give much advice. Baking soda will help, but it's not permanent. I have heard crushed coral does well, you might investigate or ask about it specifically.
On to the damage ammonia can do. The damage is lasting. The fish may make a "full" recovery in that they act normal and swim normal and eat well and breed, but it likely has shortened their lifespan somewhat. Don't beat yourself up over it, because it's already happened and there isn't much getting angry can do, other than serve as a motivation to be more vigilant.
So to sum up, it happens, think about feeding a lot less (I constantly live in the fear that I am underfeeding my fish and none have starved yet!), and also consider really turning that substrate over to deep clean it. Good luck!