The tank is full of cold mains water sitting in a warm humid room, so the room's moisture condenses on the porcelain exactly like a cold drink on a summer table. It is condensation, not a leak, and in Florida it is close to inevitable without a fix.
How to be sure it is sweat and not a leak
Dry the whole tank and the floor completely. Wait an hour without flushing. If the entire outside of the tank has gone evenly damp, that is condensation. If instead you get a bead running from one point — a tank bolt, the supply connection, the joint between tank and bowl — that is a leak, and it is a different job.
Second check: does it only happen in summer, or only after several flushes in a row? Both point at condensation, because both are about cold water refilling the tank faster than the porcelain can warm up.
Why it matters more than it looks
People live with a sweating toilet for years and it quietly destroys the floor around it. Water runs down the porcelain, off the base, and sits at the joint between the toilet and the flooring — which is the one spot with no sealant and, under vinyl or laminate, no tolerance for standing water. Soft flooring around a toilet base is very often years of condensation rather than a plumbing failure.
The fixes, cheapest first
- Run the fan longer. Condensation needs humid room air. The same twenty-minute timer switch that fixes ceiling mould helps here.
- A tank liner kit — closed-cell foam panels glued inside the tank, around twenty dollars. You have to drain and dry the tank thoroughly and let the adhesive cure properly, which is the part people rush. Done properly it works.
- A mixing valve that blends a little hot water into the tank supply, raising the fill temperature above the dew point. Effective, more plumbing involved, and it does use hot water.
- An insulated tank. Most modern toilets are sold with foam already bonded inside. If yours is old enough to be bare porcelain and you were already thinking about replacing it, that decides it.
What not to do
Do not just put a tray or a towel under it and leave it. That keeps the floor wet for longer, not shorter. And do not turn the supply down to a trickle to slow the refill — you get a toilet that takes four minutes to be usable and it does not stop the condensing.
If the floor is already soft
Press around the base. If it gives, or the flooring has lifted or discoloured in a ring, the subfloor underneath has taken water for a long time. That is a floor repair with the toilet pulled, and it is worth doing before it reaches the joists.