First if your water changes have not had any afect on the ammonia levels you test kit maybe bad. you should see a deffinit drop in the ammonia level exspecialy if you have done a 25% water change. you could do anouther 10% water change today and every 2-3 days till you find out what is happening. What snails do you have in your tank?
I am asking you this because I have white disk like snail eggs on my glass and rocks and they apear almost over night. I'll try and get a good pic of some and post in this thread. you also said that you have afilm on the water top this could be from the snails sponing and you have extra ditris that is being suspended in the water colume try and redirect your power head so it disterbs the water surfaces see if this helps to clear it up. I get this on my kids tank once in a while you can scoop as much of the floating stuff off the top.
If you have too many snails or there spon is dead it will rase your ammonia level.
don't panic go slow.

I'll check back in a while as I have to go get some shrimp and maybe my fish came in also.
Bill