how big of a fish can i have in a 10 gal

Can't say for certain, it really depends on what kind of fish you want. Some fish, although small, can be very big waste producers... and thus won't work in a smaller tank, others can be medium sized, but low waster producers, and thus will do alright in a smaller tank. Other factors to keep in mind are, potential tankmates, type of tank setup, filtration, etc. etc.

What do you have in mind?

-Richer
 
I can't see much more than a pair of the S/A Dwarfs, Cons, or Neets in a 10g. I also can't see too many shell-dwellers in any more than a pair, although there are some small enough to do that. I think that the behavior of cichlids will be exacerbated by cramped conditions, so a pair (or a little more than that for shell-dwellers would be your best bet).

P.S. Why would you tell us that your barracuda is cramped? Get a bigger tank before you get more fish!
 
Considering FW barracuda (Acestrorhynchus falcatus) grows to about 3 feet, Id say the 20 gallon is pretty small for it.
 
I'm guessing that at its final size, nothing could be kept with it, but you need to get a much much bigger tank by that point, and no tankmates for it at any point.
 
16 inches of fish is the max for a 10 gallon....but I always keep it under that just so that they are happier.
 
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