well, if u wanna get the most out of the sand, you'll have to get rid of the eel and goby..
If u wanna keep them, then i suggest u buy some dry sand... Cause your sandbed would only serve the aesthetic pourpose. Sandbeds have so much to offer, but u must set it up correctly for it to be efficient.
That means no eel or goby..thats violating the sacred rules of a deepsandbed.
I love deepsandbeds cause thats going to be my future fossil fuel source(in a couple decades).. Im designing a small scale pretolium refinery.. And guess where im drilling??