Although the tank has been running for 6 months, something has wiped out your bacteria colony or you wouldn't have ammonia. Have you recently used any type of medication in the tank. Quite a few of them will kill the bacteria colony. You say you have 2 filters in the tank. What type/size are they? If one is considerabley stronger than the other, then changing the stronger one would have removed a larger protion of you bacteria that the tank could stand.
As mentioned, all you can do is water changes until the tank cycles again. I would do a large 75% water change (what is the pH of your tap water?) to get the ammonia down as low as possible and then do smaller 25% WCs 2 to 4 times a day until you get the ammonia down below .25ppm. The reason I asked aboout your pH is that if the tap pH is considerably higher than the tank pH of 6.0 (could it possibly be lower and that is as low as your test kit goes), then a large water change could change the pH too much and would therefore not be a good idea. If that is the case, you should just do 25% water changes every 4 to 6 hours until you get the levels down.
The one positive point to your low pH at this point is that ammonia is almost non-toxic at a pH leel that low so in that sense it isn't bad. Most of the fish you have except the mollies would actually be quite at home with a pH of 6.0. And all of them can adapt to it. If you do want to raise it, get a piece of tufa rock and place it in the tank. It will raise it naturally and keep it up.
