New 10 gallon tank won't cycle

I guess it may take awhile since you only have that 1 fish in there. I have a 46 gal and it took 7 wks before the cycle was completed and I had ten fish in there and they all are still alive.

Sounds like something is totally wrong. I'm kinda new at this myself, I've got a 10 gal I'm cycling fishlessly and am on about day 18 and so far so good!
I'm just adding a little bit of pure ammonia as needed,no Cycle or anything else.
Is there anywhere else you can keep your molly? Maybe you need to totally remove everything, clean it with a mild bleach solution and just start over. Thats probably what I would do. But as I said, this is my first tank, so maybe some of the more experienced people could give you a better suggestion.

Hope you get it figured out.

I don't agree...

I wouldn't use bleach in the tank, it could kill any fish you were to put in there...

I would try to go to a pet store(with your filter) and see what they think. Don't go to the local Wal-Mart, they don't know about fish like a pet store would...

Good pet stores will be willing to take time to figure this out with you...

Have you checked the KH and pH?

relatively soft...

I wouldn't use bleach I use salt to clean a fresh water tank out. I have never had a problem with a freshwater tank cycleing I started my 20 gallon with setting it up until I had a bit of algae, and put in a cory cat then I added a algae eater
I have used this method of adding one fish then 2 then started adding a few more at a time till I had the tank stocked the way I wanted it
I would bring a large sample of water to your lfs like was mentioned a real pet store and ask for there help I would def check ph and make sure there are no bad decorations that were natural and not cured.

off scale ammonia

So, the ammonia is off the scale, you were using an ammonia detox but stopped using that. Now you are changing 40% for the past 3 days.

It seems to me that 40% water changes are not nearly enough. I suggest larger water changes, back to back if necessary to get the ammonia not only readable but down to nearly not measurable levels. And using the ammonia detox after that.

My first tank went ballistic like that, well actually it was the second tank but I bleached the filter when I took it from the first tank fearing that fish was ill and the filter was not healthy...big mistake, all those dead bacteria make one heck of an ammonia spike if you can't get them all cleaned out of the nooks and crannies of the filter. Anyhow, I had over-the-top ammonia and then nitrite spikes. One day I did 3 back to back water changes, first 50%, then I panicked when the ammonia was STILL off the chart so I did another 50+% andi t was still off the chart but pretty close to 5ppm so I did another 85% to get it down to near 1ppm. I had a few more days of two water changes a day after that, then it got better. I kept changing water daily until it was down to 1.0 max. It was a planted tank too, so things could have been worse I suppose. Anyhow, most of the fish survived until a water change with too-cold water, the last one lived over a year longer but seemed to be a bit tender in the gills. Ammonia detox stuff would have helped him I bet, had I used that.

So, I'm thinking the ammonia detox thing is probably a good thing for the fish, assuming it leaves the ammonia able to be processed by bacteria. I'd think the label would say something about that, I know Prime says that the ammonia is able to be processed by bacteria but is left unable to harm fish. Don't know about your stuff.

Most water conditioners that treat chloramine will bind the ammonia so that it is not harmful to fish, but you can't tell if the ammonia is bound or not with a regular test kit. Read the instructions, if it says Nessler based test, then the ammonia reading you get will not reflect bound vs unbound ammonia.

The combination of things doesn't add up well, one fish should not produce the amount of ammonia that you are seeing, how much do you feed your molly? excess food may be the source (but doesn't explain the absence of nitrites), with ammonia that high, nitrite should show up fairly soon and when it does, it will spike very hard most likely. this will be harder on you molly than the ammonia. The water changes should cause improvement. Have you tested your tap water? also it might be wise to take some water to the LFS and have it tested to verify the accuracy of your test kit (just a precaution) On this note, how is your molly doing? I would think there should be obvious signs of stress.
Additional and/orlarger water changes should be done to get things into a cotrollable range. Bio-spira might help. Essentially if the water changes, reduced feeding, and testing don't show a change, I would tear down the tank and clean it. I personally would use chlorine bleach,Salt also works, but bleach kills everything, salt may or may not. rinse well and test for chlorine before putting fish back in. Chrlorine is easy to take care of with conditioners if you do have residual bleach.

What type of media are you currently using, what is the air pump used for etc.