knife mouths and tank mates

[equidae9854]

I've got a 12" CK and a 12" RCK.
I was wondering if there was a certain rule of thumb as to how wide they can open their mouths. I was just wondering what size tank mates would be a safe choice for them. I have a 180 gal with some synodontis cats that are about 5-7" and round and fat, but I'm too scared to try mixing them...
Also, if they can't be in with my other current fish, I do have a spare 125 gal that I could keep them in, but I will also definitely consider letting them go... Just curious, how much do these fish go for at this size??

 

[benzjamin13]

5" would worry me a bit. The pectoral and dorsal spines would do some major damage to the Knives if they decided to go for it. 7" would be fine. As for selling them, you'd probably get under $25 for each.

 

[equidae9854]

Thanks. *sigh* oh the decisions...