I use 2 1/2" pvc tubing, perfect for fire eel and plenty of room for him, ornate bichir also decided to colonize a tube... much less room for her. Sometimes I am worried she will get stuck, but she navigates seemingly fine, watching her wiggle back and forth to move fractions of an inch at a time is kinda silly, but there is no deterring her, she loves to hide in tight fitting places. The fire eel I have often seen sleeping on its side in there, kinda cute... reminds me of how clown loaches sleep. Definitely get tube for your eels. I don't know if adult eels burrow as much as the elvers do, but mine stopped doing it (probably because substrate wasn't deep enough. Tubes are an eels best friend though.