mid water fish for a ray tank has always been a problem
the way i look at it their are lots of fish that can go in a ray tank but dont think it will last forever
the only fish i know for sure will be safe are tigers (dats) and a few large catfish (not plecos) i have never tryed clown loach but i have been told they work well as they have a sharp dorsal fin like tigers and the rays dont like the feel of it
the only problem with clown loach is they are prone to white spot and if they get it when kept with rays you wont be able to treat it
as for the flag tail you can try to keep more than one in the same tank but i can nearly garantee you that it wont work one flag tail per tank if you dont want ripped up tail or dead flag tails
their are a few people on other sites that think that flag tails have sucked and killed their rays but to me thats rubbish as no healthy ray would just lay their and get sucked to death
my flagtail did suck on my rays disk a few times but the rays just push it off with no damage or stress to the ray at all my ray cloacked and chewed the flag tail a few time after he had been sucked buy the flagtail now the flagtail stays well away from all of the rays
flag tails chould be a problem if the ray doesnt have enought space to get out of the flagtails way
discus wont last long
the uaru.f i had worked well but the normal brown uaru.a that you normally see for sales seam to like pecking at the rays disk and even go for the eyes
silver dollars,red hooks or black bar dollars also work well
but at the end of the day tank mates for the mid water in a ray tank is just a temp thing they will all end up as feeders at some point