just get some temporary tankmates like oscars, there are a ton of oscars out there cheap and it'll give ya something for a little while, and won't cost ya when they go missing
I kept mine with arowanas but the most important issue is the nex tank because them will be big in 6 months and huge in a year. Love the darn things but too bad they don't have a version that only grows to a foot instead of 4 feet. My largest was over 30 inches long in 300 and that was not really big enough. Need lots of good filtration.
Your keeping a rtc and think oscars are messy? Wait till your rtc is over 2'-3'. I consider my oscars to be miniscule compared to the catfish they live with. Haven't had a single one go missing unless I put it in with the big boy.