Eel is in the sump with a bunch of LR.Royal gramma with an attitude! lol. If already well adjusted, should be able to feed at one particular area (a dining table) as I have done with many fishes including seahorses. You can even shut the circulation when feeding thus less leftover food decaying somewhere in the tank but make sure to turn the pump back on. Of course this is until tank settles. As far as your NH3 number, it may be on the way down. When properly done, you may not see any increase in NH3/NO2, perhaps bit of NO2 but NO2 in saltwater is virtually non toxic to fish.
Eel can eat as much as you offer but just feed sparingly until tank is well settled.
Why on earth do you have an eel if this tank is going to be a reef? Not that you can not but wouldnt you rather have more smaller fishes and shrimps/inverts (yummies for an eel) in the tank?
I know of many who kept triggers, puffers and even bigger eel such as Tessalata in 120 Reef with larger tankmates, Harlequin, Parrot, etc etc. and I was one of those crazies!
I am really concerned with the fact that you dont have easy access to sump. There will come a time when you have to get in there to service or perform maintenance.
What kind of return pump(s) and PS are you running?
When I was doing some extra plumbing work and dropped something in sump, I found out that skinny people, like wife who rather not have hold in wall, can easily get their entire arm in sump area, which said wife will be doing when needed ^^.
Don't know what Kind of return it is, but I remeber it was like 2500 Liter/Hr. So far no problems.
PS? protien skimmer? I have a euro-reef skimmer, which pumps like 350g/hr.
And I jsut finished my ATS so it'll be put outta service soon.