I have just purchased a Maxi-Jet 900 to replace my original pump on my 12gal nano cube. I pulled out the original and noticed the 900 to be much bigger, wider and taller than the original. It will not fit in any way into the 4th chamber or the 3rd chamber. I have read that this is a common upgrade with not a lot of difficultly. What am I missing? Any help would be appreciated.