I am a scuba diver. Garden eels are found in sand flats in about 30 plus feet of water. They stick out of their holes about half way and sled in when a diver approaches. I would not.mix them with a snowflake eel. Ithe snowflake is more of a denizen of live rock. It is also considerably larger than garden eels and might think of them as a nice snack. If you go with garden eels, I would not think an under gavel filter would be a good idea. I think it would mess with the dynamics of the eel's burrow. If you like burrowing creatures, the pearly jawfish might be an alternate choice. They have nice color. They like burrowing in sand mixed with rubble. I once had a couple. They would carefully select pebbles and arrange them at their burrow's entrance. The problem was that they stole each other's prized pebbles. It was hilarious. If you are set on an eel, I think the snowflake would be better.