No not a specific number. People on the forums have me scared that it will be too much weight in an apartment so I'm having to lower the weight as much as I can. And isn't a 240 gallon like 2000+ pounds? And I don't think the issue is the weight itself. It's the weight over a period of time. Correct me if I'm wrong.
I'm saying the difference, while not negligible, isn't putting you that low. You're only talking 500lbs. 180g of water is still 1500lbs (rounding). 240 gallons of water is about 2000lbs (rounding). All other things being the same (since you're using a 250g tank), you're still going to be edging towards 2000lbs once you put that water in a tank, on a stand, with filtration and livestock. However, I would certainly agree that 500lbs sitting there all the time is different than 500lbs standing in front of the tank.
If you're already spreading 2000lbs across 5 or 6 joists (400lbs min), I don't see what an extra 100lbs (per joist) is going to do.
Doing some quick googling, a 2x10 floor joist spanning 13.5' can support a dead load of 1200lbs - which leaves some leeway for live load (people standing there). Deflection would be greater in the center of the floor, obviously, then it would at the supports (near the wall). So, technically, 2 of those would support the tank lengthwise assuming they weren't supporting something else (like couches, counters, refrigerators, etc.). So, we'll say you fill it the brim and add a bunch of other stuff and get to 2500lbs. If you are smart and place it so that it spans multiple joists, then you're spreading that load across at least 5 joists. That is 500lbs (450 if you set it on six real perfect like) per joist, well under the weight rating for the joists. Obviously, if you're using 2x12s or the span is less or greater, the load can change quite a bit. Depends on your building codes and what they had to use. For all you know, in an apartment, you have a steel i-beam running right under your floor, that is holding the whole building up.
Of course, I am usually full of crap and nobody is going to sue me if 240g of your water end up in their lap while they're watching South Park re-reuns...