400.00 what to do with?

I'd put it in the bank for a while...

one word... DISCUS!!! *grin*

you deffinitely need to check if your appt can handle that... it depends on your construction a 75gallon is nothing in weight compared to a 180... its less than half of it so it is in no way a indication of what your floor can handle. First and foremost figure that out because 180gallons on the floor going into the next appts roof will surely cost more than you have coming lol. With regards to fish, if you could have a large tank like that I would probably do a river theme with darker water and keep some cool larger fish, like a lungfish perhaps?

If your apt is not on the ground floor, you'd really be pushing the envelope and taking a fairly large liability risk if you're thinking of something the size of a 180 gal tank, as AquaticAustin pointed out.
And as aXio said, a discus tank would be a great way to spend your available $.
Find a good 75 gal set-up on Craig's List, buy yourself the necessary equipment, and get a half dozen nice-sized discus.
If that appeals to you, I'd be glad to help you out along the way, in any way I can.
Food for thought:
Here's my planted low-tech discus tank, about the same size as 75 gal, for you to have a look at:
http://s1105.photobucket.com/albums/h357/discuspaul/Sept2011

Thanks,
But the big question is - does it appeal to you - would you consider trying it ? You'd love it.