Back when I was 8 and taking care of my dads 30 gallon tank. We had a rope fish and an elephant nose fish. They both died around 6-8 years of age and the elephant got roughly 8 inches. It just depends on feeding habits and water clarity. I think 14" is possible if it lives for a very long time under good conditions. Btw what is a nim.(also I probably would have said something to my dad about stock and tank size because a 30 is small for elephant nose but back then I knew nothing about the fish, he picked out and bought it and I took care of it).
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That's awesome how your dad is into fish as well, I am pretty much the only one in my family who cares about fish, excluding my Grandpa who passed away when I was 11 or so. I know I wish I could go back and change some of my stock choices as well when I was younger, I was clueless though as well, as we all were when we started in the hobby.

Oh and Nim/Nimmy (aka Nimrod, named from this adorable critter from Surface http://sharetv.org/shows/surface/cast/nimrod is a Senegal Bichir, he's only about ~3.5" right now, in my 29 gallon as my 75 gets cycled.
I'm changing stock again, now that I am running into my typical problem, I only seem to want fish that are mainly bottom dwellers. Especially now that I have discovered the adorable peacock eel.. sigh lol.
So the new stock will be
~Nim
~3 ropefish
~1 peacock eel (not really a *monster fish* but they are darn cute!)
~Some sort of top/midweller, currently considering leopard ctenopoma (I'm guessing they aren't MONSTER FISH as well?), just because the gar like characins are too hard to find/too big/too difficult it seems. Plus, they have big "bug eyes" and leopard spots, which I adore as well.
I'm thinking of just foregoing the knife fish at this point and time, because I would just be settling for the brown ghost knife when I am in love with black ghost knives, and the above fish that I am getting now.