There is no mature media. I've had 3 ottos in my 60L for about a year now, although it was cycled when I added them. Only problem I've ever had with them is that one of them kept getting stuck in things, first it got stuck in the cap on the bottom of a cheap heater (to stop it smacking against the glass) I saved it from that fortunately. Unfortunately I lost one of the three otto's a couple days ago, was looking for them and eventually found the third, dead, stuck in a small hole on the underside of a piece of driftwood I bet it was the same one .
Surely 2 ottos in a 450L body of water wouldn't cause an ammonia spike? Maybe I'll find some other random fish to throw in there. Thanks for the heads up.