An autofill is a must have for any koi pond for a number of reasons. Evaporation will always be a problem. The more surface area that your pond has the quicker your water will evaporate. An autofill is a quick an easy way to counteract this. We recommend turning off your autofill once a month to check for a leak. The exterior autofill as an overflow system. This means that in the unlikely event your auto fill fails you don’t have to worry about the safety of your fish. Any excess water will spill out the over flow and not into your pond.