An 18,000 gallon pool is best heated using a 200-300 BTU heater. Although a smaller heater may be capable of heating heat an 18,000 gallon pool, it's slower to heat the water and needs to work harder.
A heater larger than 200-300 BTU's may be recommended if the owner heats the pool intermittently. Larger heaters heat the pool faster, and for this reason may be more desirable for intermittent use. A smaller heater, such as a 100-200 BTU unit, may be used if the thermostat must maintain the pool at a consistent temperature. A pool cover can help the heater work more efficiently.