Few suggestions / ideas / pointers for you to perhaps consider:
Snails and leaves along are not going to give much ammonia, depending upon how many snails of course. Are you feeding the snails? What snail type / size / number? You probably need to add some fish food to either create ammonia directly, or else for the snails to eat and then create ammonia indirectly.
API test kits are generally unreliable between 0 and 0.25ppm readings. Really hard to distinguish that slight lime green-ness. Many people with well cycled tanks report getting 0.25ppm readings (me included!). So 0.25ppm coudl easily mean zero, and an actual low ammonia reading could easily be missed if you don't hold the test tube just right against the card in the right light.
If it's gravel, any particular reason you didn't wash it before adding to the tank if it was "mucky"? Not sure if you mean this was good much or bad muck?
I'm guessing your tank *isn't* cycled yet given it's age (2-3 weeks?) and that there hasn't been much ammonia supplied. Have you been adding any bacteria products, or just waiting for the bacteria to colonise naturally? I'd say 3 weeks would be pretty good going to get a tank cycled with lots of bacteria and ammonia added. Perhaps more likely that your cycle hasn't really got going yet?
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