Spathyphyllum are water-loving plants; you continue to stress the plant by taking it to the wilting point. Don’t be surprised if they continue to fail with that regime.

Try a more evenly moist approach with monthly feeding and see if your plant doesn’t improve.

Though PLs are often found in riparian settings, and many people assume they need a wet soil to thrive, they prefer a damp or moist soil, never wet or soggy. The black spots can be from over-or under-watering. One limits water uptake by virtue of thew fact there is little or no water in the medium that roots can absorb, the other, over-watering, limits root function due to the limited amount of oxygen saturated media can hold. With the oxygen supply limited, roots lose their ability to function normally which leads to damage on the plant's most distal parts, leaf tips and margins. Your plant can then actually die of thirst while its roots are awash in a sea of plenty.

If you're waiting until the plant wilts before you water, you're waiting too long. Drought stress affects roots well before leaves show symptoms. Dry conditions cause fine roots to die, and the plant's chemical messengers to signal the plant to stop producing additional foliage until regeneration of new fine roots adequate to support any new growth. This robs your plant of a good deal of potential in terms of growth, vitality, and eye appeal. I'll leave something I wrote below, about using a wooden 'tell' to 'tell' you when it's it's time to water. Getting watering intervals right is a key element of any plant care regimen and will help you maintain your plant(s) in good health.

Are you fertilizing regularly? If not, would you like to talk about an easy nutritional supplementation/ fertilizing regimen?

Using a 'tell'

Over-watering saps vitality and is one of the most common plant assassins, so learning to avoid it is worth the small effort. Plants make and store their own energy source – photosynthate - (sugar/glucose). Functioning roots need energy to drive their metabolic processes, and in order to get it, they use oxygen to burn (oxidize) their food. From this, we can see that terrestrial plants need plenty of air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support the kind of root health most growers would like to see; and, a healthy root system is a prerequisite to a healthy plant.

Watering in small sips in order to avoid over-watering leads to a residual build-up of dissolved solids (salts) in the soil from tapwater and fertilizer solutions - which limits a plant's ability to absorb water – so watering in sips simply moves us to the other horn of a dilemma and creates another problem that requires resolution. Better, would be to simply adopt a soil that drains well enough to allow watering to beyond the saturation point, so we're flushing the soil of accumulating dissolved solids whenever we water; this, w/o the plant being forced to pay a tax in the form of reduced vitality, due to prolong periods of soil saturation. Sometimes, though, that's not a course we can immediately steer, which makes controlling how often we water a very important factor.

In many cases, we can judge whether or not a planting needs watering by hefting the pot. This is especially true if the pot is made from light material, like plastic, but doesn't work (as) well when the pot is made from heavier material, like clay, or when the size/weight of the pot precludes grabbing it with one hand to judge its weight and gauge the need for water.

Fingers stuck an inch or two into the soil work ok for shallow pots, but not for deep pots. Deep pots might have 3 or more inches of soil that feels totally dry, while the lower several inches of the soil is 100% saturated. Obviously, the lack of oxygen in the root zone situation can wreak havoc with root health and cause the loss of a very notable measure of your plant's potential. Inexpensive watering meters don't even measure moisture levels, they measure electrical conductivity. Clean the tip and insert it into a cup of distilled water and witness the fact it reads 'DRY'.

One of the most reliable methods of checking a planting's need for water is using a 'tell' (more reliable than a 'moisture meter'. You can use a bamboo skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm) works better. They usually come 48” (120cm) long and can usually be cut in half or in several pieces, depending on how deep your pots are. Sharpen both ends of each tell in a pencil sharpener and slightly blunt the tip so it's about the diameter of the head on a straight pin. Push the wooden tell deep into the soil. Don't worry, it won't harm the root system. If the plant is quite root-bound, you might need to try several places until you find one where you can push it all the way to the pot's bottom. Leave it a few seconds, then withdraw it and inspect the tip for moisture. For most plantings, withhold water until the tell's tip comes out nearly dry. If you see signs of wilting, adjust the interval between waterings so drought stress isn't a recurring issue.

Al