ask johnptc or arapaimaguy, tlkmnd, necro. they'll give you your 5 feet. and even 3 feet is way bigger than most of us can care for. I'm an a-hole since I keep a tsn in my 300, you on the other hand actually recommends a guy who is already severly overstocked (given those fish reach adulthood) to get a freakin RTC. The same guy who wants to buy a finch just to feed his aro?
and even IF you were partially right? have you seen an RTC maxing out below 30"? his tank is 2 feet? please do explain how you got this to add up? Everyone says they will upgrade. but money and space is a constant issue.
for your question OP, I'd get some sort of bichir, maybe an endli, or ornate, looks good, will get you a good amount of money back if you decide to sell it. bichirs are always fat even if you fed 3 days ago, they always find something edable.
but if you really want a cat, go something along the lines of clarias batrachus, they eat everything. and if you are in the states try a jur or lima, tig is pretty (and) expensive.