Depends on the fuge. They usually have thumb screws and pipe out tend to have a bit of clearance. If you can find someone who does custom builds you can ensure it will fit nice.
If going for a hang on fuge word of advice. Make sure your algae, ie chaeto is prevented from entering the return pipe. It happened to me and it emptied about 15 gallon on to floor whilst i was on holiday (of course whilst on holiday). Pipe was part blocked and input was greater than output, as such fuge vol raised and poured over sides. It only stopped when pump became exposed to air which at the time was about half way down old style rectangular aquarium.
I do nothing for nitrate in tank and haven't tested water parameters in about 8 months. I ought to check it really. Mind you i can tell when water isnt up to par by looking at it.