Ok, I read through your information, and I can see a number of problems that are contributing to your issue.
First and foremost, that fish needs to go into a quarantine tank if you're hoping to save him. He is suffering from nitrite and nitrate burns to the skin, lack of oxygen, and low spg/salinity. Add to that it appears he also is dealing with a bacterial/fungal problem, which was probably brought on by the weakening of his immune system due to the other issues I just listed.
How to save him? Can you save him? If you can get him into a quarantine tank asap, maybe. Salinity should be reading about 1.023 - 1.025... you're a touch low... but to him that can make all the difference in the world. Powerheads alone won't account for making up oxygen content in the water... so unless it's pointed at the surface and you're getting bubbles, it's probably not going to do much good for oxygen content... and with those fish at those sizes in that size of a tank... I'm surprised the others aren't suffering already too. A 12 inch snowflake eel does not belong in a 20 gallon tank alone, much less with tank mates... and aggression isn't the issue. You have 1/2 the amount of live rock that size tank needs, so biological filtration will be low, and it is showing up in your water paramters.
Any ammonia or nitrite is toxic, and nitrate over 40 is also highly toxic. Because nitrate is the end product of ammonia breakdown until the denitrifying bacteria set in, a nitrate reading of 100+ can't happen overnight... that takes long term excess waste levels to achieve.
After reading about how long the tank has been set up, I'm willing to bet that it's still working it's way through the cycling phase. Most marine aquariums will take about 8 - 12 wks to cycle fully, with enough sand and live rock in place... add a heavy popluation of fish (which you have) and messy eating habits (which those fish are known for) and you will only make the tank cycle harder. Levels will go off the chart instead of staying managable, and it will take longer for things to level off. For what you have in fish right now, at their sizes, they should be in at least 55 - 75 gallons already.
Oh, and deepen that sand bed to 2 - 3 inches. You currently aren't going to be able to keep a stable tank with lack of live rock and sand needed for bacteria to culture, and no filter media in the world is going to be able to keep those conditions under control long term. That is like putting a band aid on a severed leg and waiting for it to heal. You should be doing daily water changes of about 10% to bring down that nitrate level and help increase oxygen levels. I didn't see a calcium reading posted... this is also vitally important to the survival of your fish. I would suggest checking that asap, and working with liquid test kits instead of strips. Strip tests are extremely inaccurate and worthless when you really need to know what's happening in there. I have seen so many tanks crash due to relying on test strip readings, with total loss of animals!
How to fix the problem with the trigger knowing all of this: Get him into a clean quarantine tank with proper spg/salinity, keep him clean, give him a few days to recover from the stress of moving and the poor water quality of the main tank, and if he's survives that long, then we could consider medication for infection. You don't want to dose meds into the main tank with the other fish unless you want to cause the other fish problems/harm.
Sorry I can't offer you better news, but I say it as I see it... and my goal is always to help the fish.
If you only use it for a couple of days, try using polyfilter in the filter to drop the nitrate levels, and nitrite, too. Longer than a few days at a time and it can drop the pH in the water, so don't abuse it. You can find it online at Dr's Foster & Smith, and if you need it, I can post a link for you to find it. I would strongly suggest getting the big pad, use 1/3 of it in your filter all at once, and then save the rest for future use until you can get these fish properly homed. Again, it's a bandaid, but it works better and faster than any other media available.