Just how did you do that 20 gal water change? For one thing, you first have the water ready with the correct salinity level and you then cut the power to the tank and with a hose from the lfs, you siphon out the correct amount of water your making the change for and then pour slowly the water and then turn back on your tanks power.
Its a mystery on how you done that loosing all your other fish. for once your water change is ready to perform, it only takes a few minutes for I do a 30 gal a week on my two tank, one system and I use a strong pump with a one inch hose to first remove the 30 gals from the tanks and then I wheel it to dump and then I pump it from one Rubbermaid to the one I on wheels and then I wheel over to the tanks and pump it into the tanks. And after that I turn the tanks power back on.
How did you do that 20 gal water change?
Now right now, your changing your story on you water changes, you said before you do every two weeks a 10 gal water change and now your saying something different
I will get a the water parameters first thing in the morning. I'm unable to put a sump on this because of the location of the tank. But I will be upgrading to a 150gal that will have a sump.
Now you already have some HOB things, right, you can still place a sump on this tank. You can get a 30 gal tank or larger and use a siphon oveflow box and a return pump that will mostly match the flow rate of the siphon overflow box. There be an old saying to where there is a will, there`s a way. besides, by the time you get this 150 together, you properly be too late for the eel. as is, you still not provided a picture.
Your tank is a 75 FO system, I believe that two rio 900 will do, one on both sides with one few inches below the water line and on the other side, half way down. And if another is needed, you can use one of the 300 that you now have for this also depends on how your LR is setup.
The actions your discriping of the sfe, can be most anything as the problem, is the eel acting all else as normal? You had this eel for three years, you must had learned something from how its normally behaves, You can see the eel, no one else.
PS.
I`m also beginning to think that the only problem was that the eel not feed the last time, meaning there is nothing wrong with the eel. If so, I not needed to answer this for then the only problem you had is your setup and how your bio load is to much.