I think the rule of thumb for heaters is 5 Watts of heat for each gallon.
So, maybe use (2) 400Watt heaters...
I've got a 180 on the 3rd floor running parallel to the floor joists
This is just a thought Jester but I was under the impression that running the long side of the tank across multiple support joists would be better than running it parallel to them. How long is that tank anyway?? When running the tank parallel to the joists you only have 2 joints for support... Typical floor joists should be 16 inches apart, so if your tank is 4 feet long then you would be distributing the weight across 3 or 4 joists and if the tank was 6 feet long you would be distributing it over 4 or 5... much better than just 2!
And to be on the safe side, figure 10lbs/gallon of tank. That should give you a good safety factor for the water/tank/sand/rocks/etc...