I have three succulent plants that I believe are Gasteria. One leaf in one plant has suddenly shrivelled and turned pale greyish brown. Sorry, no matter how I try, the colour in the picture is not very accurate. It looks a lot unhealthier in reality than it does in the picture.
The plants are offspring from a large mother plant and have lived in ordinary indoor potting soil for one to two years (the mother plant lives in soil too). They have always been on a north-east window sill, and if they get any direct sun, it's at most one to two hours in the early morning.
I water them every one to two weeks, and the soil dries out completely between watering. The water is ordinary tap water which in our place comes from a lake. Because I see fungus gnats every now and then, I add Bacillus thuringiensis israelensis tablets to the water about every second time.
The plants have looked relatively healthy ever since I got them. The one in question has been repotted about half to one year ago, and I saw back then that it had made extensive roots. Another of the three plants is growing its own offspring above the bottom most leaf.
There has been no change in watering that I'm aware off. I watered the plants two days ago and did not notice anything unusual, thus I believe this leaf has shrivelled and turned pale greyish brown relatively suddenly.
What could be the cause?
Additional information in response to comments:
The pot is the type normally used for expanded clay pebbles. To water, I remove the inner pot so that any excess water can drain from the many holes of that inner pot. I'm pretty sure there's never standing water.
Some months ago, I tried to fight fungus gnats with nematodes which required the soil to be kept moist for a few days, and these plants did not seem to suffer from that.
Our tap water is said to be the best quality for drinking. It's all I have ever used for my plants. Details are available here (the page is in German but the PDF is available in English).
The bacillus thuringiensis israelensis are from the gardening department of the supermarket, their website is here. One tablet is to be dissolved in 1 litre of water.
My watering can is about 0.8 litre, might such an overdose of bacillus thuringiensis israelensis be harmful to the plant?