The clear thing in the middle is a fry box, it has grates so the bigger fish can't get in. The driftwood is, yes, a cave for the convicts. I have an Eheim Classic 2213 I believe, but it is by far way more complicated to service than my new Rena FilStar Xp3 lol. There are about ~100 fry, who are growing fast now that I have frozen bloodworms, brineshrimp and mysis. The airstone in the middle is by far not their favorit toy, they always push it out lol.