Hmmm, maybe someone knows a better way but to my knowledge the adhesive used to bond the acrylic pieces together actually "melts" it to each other. A permanent bond I guess you could say, at least I have never had or heard of one of these joints failing. My only thoughts are you would have to get in there and physically cut this portion out using a saw or even a Dremel tool perhaps....
I can only assume the purchase of this tank must have been in a manner that somehow prevents you from returning/exchanging it for the size you really need/want.
Sooooo, if you really need to take this part of the tank out I would definitely say cut it out as opposed to trying to force or break it out, my guess is that joint is stronger than the surrounding material and there would be a big risk of cracking the tank if you tried removing it by force.
Good luck!
