Depends. Are we talking Electric or Hydronic (Hot Water) radiant floor heaters? I don't think electric would be strong enough to heat an entire 1,000 gallon tank, but if it was strong enough, the electricity bill would be crazy high. Hydronic radial floor heaters could work, but it would probably work better to place the heating tubing in the sump, rather than under a large aquarium. TheFishGuy use to have his 1,600 gallon tank plumbed that way. Had the Hydronic tubing coiled in his sump, which where connected to a pump and thermostat, and plumbed that to his household water heater. It just recirculated the hot water from his hot water heater, when the thermostat kicked in.
Electric substrate heating cables have been used in planted tank for years. The heat helps slowly circulate and distribute nutrients to plant roots, but these heating cables aren't not nearly powerful enough to heat the water in a big tank. They simply aren't made for that purpose.
I've heard of people using waterbed heaters underneath bare bottom glass tanks with some success. Some have tried using waterbed heaters underneath pond liners also, but the radial heat usually causes condensation/moisture problems underneath the liner.