Hello --
How long have you had the tank running? That's quite a lot of fish. One thing to bear in mind is that "ammo chips" absorb ammonia, and in doing so slow down the bacteria from getting established in the filter. Personally, I prefer to use bacteria for biological filtration (i.e., removing ammonium and nitrite) and leave chemical media for specific things like removing nitrate and phosphate.
A biological filter takes at least 6 weeks to mature. If you can, leave the ammo chips for a while, and carry out regular water changes to prevent the nitrites and ammonium from getting too high. Since you're measuring some ammonium, that means you have inadequate filtration -- whether that means biological (bacteria) or chemical (ammo chips).
You have quite a lot of fish for an immature tank, so definitely don't add anything for a while.
Fiddler crabs need brackish water and are amphibious. I'm not sure they are going to last long in this aquarium. Get rid of them while they're still alive if you can.
Fire eels are sensitive, large fish that simply aren't easy to keep. They are prone to bacterial infections, perhaps because they scratch themselves when trying to dig into gravel. Then need big tanks with mature filters and a soft sand substrate. They are also very predatory.
Cheers,
Neale