KM,
For cartridge filter size, many here will divide your gallons by 66.
18,000 divided by 66 equals approximately 275, best choice square footage of cartridge filter.
Minimum would be 18,000 divided by 75, equaling 240 square feet of filter, so your smallest would be in the 250 square foot range.
Maximum would be anything over 275 square feet for your pool; (the closest next available size is 300 square feet -- what you have), along with any size larger which an owner would choose, recommended for less frequent cleanings.
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Perhaps you might find out if your pest exterminator can apply a solution to the palms and pots to kill the buggers!
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FROM THE INTERNET, courtesy of General Pest Control, Cleveland, OH:
Fungus gnats live and breed in soil that is damp and rich in organic content. Stir the soil of these plants with a stick and smell for a sour odor. A sour odor signals the presence of fungi in the soil which usually results from over-watering. The habitat of fungus gnats is not limited to indoor plants. Outside, fungus gnats may be found in accumulations of dead leaves, rotting firewood, compost piles, and in heavily mulched areas that are shaded and constantly over-watered. Infestations have also been found in walls or ceilings where an undetected leak was present, in pet feces at the bottom of cages, and in the drip pans under refrigerators.
The key to control is finding and eliminating all of the breeding sources. Careful inspection is important. Look for both large and small potted plants. The eggs and larvae can be killed by drying out the soil. The larvae tend to live in the top layer of soil. Stir or overturn the top two to three inches of soil to speed drying. If the top layer of soil remains dry the larvae will die off and the adults will not have a suitable place to lay more eggs. Adjust the watering schedule to prevent the soil from becoming too moist. Conversely, be careful because too little watering may cause leaf drop and/or weaken the plant making it susceptible to fungus gnat larval damage. The soil may be treated with an appropriate potted plant spray that is commonly found in supermarkets, garden centers, and florist shops or by a licensed plantscape specialist. The common method is to use an insecticide as a drench so that the first one to three inches of soil are made wet. This kills the larvae and interrupts the breeding cycle. Alternatively, a bacterial soil drench containing Bti (Bacillus thuringiensis), commonly available as Gnatrol or Knock-Out Gnats, may be applied.
Once the larvae feeding in the plant soil have been killed, the adult flies will die off in a week to ten days. The adult flies may be trapped by using yellow sticky cards placed close to or directly on the surface of the soil. Adult fungus gnats may also be captured on the sticky boards inside of insect light traps. In severe situations, and only as a last resort for indoor infestations, it may be necessary to treat localized areas with an aerosol spray containing natural pyrethrins.