If I may play the devils advocate. Yup! That looks Great. But in 6 months its going to be downtown funky town. covered in corraline, green algae, cyano and gunk. Thats its purpose. to keep that stuff out of the dt.
were it a remote refugium and not plumbed inline, that would be different. But one of the biggest mechanical advantages of a sump is, its a gravity filter, all heavy particles go down and cant come back up, so all the funk from the dt goes there to be processed and or skimmed. Like a lower bowel. So putting a fish in there kinda imo defeats the purpose as it adds to the bio load your trying to clean up after with the refugium and the fish is going to live in a high bacterial environment filled with decaying organics. Curious most think seahorses for a fuge as Ponies are highly prone to bacterial infection and usually prefer a lower temp than reefs.
Also with luck that chamber will be choked with macros like mine is and its not a very fun place to live unless your a pod.
An alternative to fish would be something more accustomed to a high detritus environment. Brittle star comes to mind, an unusual crab although some(many) eat algaes., sea hare once its established etc.
and yes, I know this because I built a beautiful 30 tall display refugium, and now its an amazing display of funk and a lifesaving example of the efficiency of refugiums . when my skimmer died while I was gone for 6 weeks and the talbot dug holes in my 18 mo old dsb, the chato and macros exploded so did the sps gorgs and clove polyps. and I never got cyano in the dt. Its filled with amphipods copepods isopods, mini brittle stars, feather dusters, stomatella limpits collonista snails, bristle worms, sand worms and mysis shrimp.
Sp it's still a display fuge, just displaying something a bit more interesting than a fish imo.
my .02