Probably less than a typical warm tropical aquarium, Doc. Keeping the lobster tank at 65°F while the room is 72–75°F does not, by itself, increase evaporation. In fact, the cooler water has a lower vapor pressure, so it generally evaporates more slowly than 78–80°F aquarium water.

The big factors will be:

• Open top: more evaporation.
• Strong surface agitation/skimmer: more evaporation.
• Fans blowing across the tank: much more evaporation.
• Low room humidity: more evaporation.
• Covered tank and humid room: considerably less.

For a 150-gallon tank at 65°F, I wouldn’t be surprised by something on the order of perhaps a few gallons per week, but the tank’s surface area and whether it’s covered matter far more than the 150-gallon volume.

One important lobster-tank point: evaporation removes freshwater, not salt. So replace evaporation loss with fresh RO/DI water, not saltwater, or the salinity will gradually climb.

And there’s a funny little wrinkle: because your tank is colder than the room, under humid conditions you could simultaneously have water evaporating from the surface while atmospheric moisture condenses on the outside of the glass and cold plumbing. Physics being unnecessarily complicated, as usual. 😂