^ I second the Seachem Stability. Since it hasn't been mentioned, before you dump your tap water into your tank when doing your next water change can you pull a sample after you dechlorinate it and test that water and post the results. It has been noted some tap water has high levels in it. This would eliminate one idea at least. Also, test kits can get screwed up easily and there may be some error in the kit, how you are reading it, or something else. You may want to take some tank water and dechlorinated tap water over to a LFS and have them test it too just for a second opinion (and it's free). Make sure not to mix the samples up.
As already mentioned, stop changing your filter pad, in fact, since I run the exact type of filter you have (only I have the 75) I'll add a couple things. You should be able to slide another filter pad into the filter box after the first one if you take the stupid "bio-block" thing out, it will fit flat against the exit into the tank. I threw the bio-block away when I bought the filter, all they really do is slow the water down to keep splashing to a minimum - I like the faster water flow and it does help. I also picked up some rigid wide pore sponge - like a 50 ppm and cut it to fit before the stock filter pad to add some bio surface - if you want to go cheap and easy you can probably fit two scrubbies in there and you can find them in 6 packs for a $1 at Dollar General - like these -
http://images.google.co.uk/imgres?i...bers&start=40&ndsp=20&svnum=10&hl=en&lr=&sa=N
My personal opinion here, at this point I would put the fish in a one gallon pitcher with tank water, turn off the heater and filter and add the additional media to it, take out the decorations and put them aside (don't wash them), then stir up the gravel really well - you have not removed water yet. Once all the gravel is stirred up then empty half the tank or 75% if it's bad or ammonia is higher again. The gravel should be about 1 1/2" or so, enough to cover and hold your plastic plants but not much deeper. Thick gravel is harder to clean and unless you have live plants it isn't needed. Put your decorations back in and refill with treated water (treated with Prime for dechlorination and Stability added to it as directed for new tank setup on the bottle). Turn your filter and heater back on. Over the next few hours your filter should clear up the water. Think of the dirty water as food to good bacteria in your filter. You just added bacteria cysts to the water (Stability) and fed them, they should start kicking in. Make sure you add bacteria every day like it says to, I think it's for a full week. You can add Prime to knock down ammonia too, but you need a little to get the cycle started too, so keep watching it. I would leave it three days before doing any water change again unless it was testing high, in which I would treat with Prime first and see if that will knock down enough. After three days I would try three gallons a day water change if it isn't really high in ammonia. Don't mess with the filter at all for a month unless it is clogged and then rinse in tank water when you do a water change.
Also, what temperature is the tank at? What PH is it?