1. Eels came from Nemos, He gets them a few times a year.
2. Yes I Qted them but only copper. I did not put them through prazi after hearing they dont do well in with that. Bring the copper up slowly, I did it over a week.
3. Nope, No concern at all. In all my reading I came across a post talking about them pooping. They actually come out of their hole to poop so there is no buildup in the sand from them. I dont see any bio load from them or the sand bed simply because I feed 8 times a day and the system is pretty big.
Here are a few other things you should know. I am down to 4 of the original 8, one did not want to eat and 3 escaped through the overflow teeth. Here is the big one. They did not stay in the can. 3 of them moved around the tank for about 5 months. I currently have 3 in the can, one is behind the rockwork somewhere and the others found their way back to the can after roaming around the tank. I guess they finally realized where the deep sand was.
When you QT them, do it WITH other fish as soon as they are eating. I did mine by themselves to get them to eat and then left them alone. They were fine when I came around but once they were in the tank the fish scared them for a good week before they started eating again(one never did). They get used to the fish fine after a few weeks but if I had gotten them used to fish while in QT I think my loses would have been much lower.
Getting them eating was oddly not that hard. For some reason mine wanted nothing to do with live brine but loved fish eggs, cyclops, rotifers and just about anything else small and frozen. It took me about 3 weeks to get them eating regular foods and not be scared of me.
Good luck!