I just noticed my cactus has these dark spots and it seems to have passed it on to one more of my plants.
Does anybody know what is it and what I can do to save them?
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It seems that this is a fungal disease. It can cause over watering or poor drainage. Keep the infected plant away from the other aloe plants. If you see brown spots on a few leaves cut them from the stem and burn. Never put the infected leaves on a compost pile. Then it can spread further on other plants. Finally change the potting mix.
Change the watering pattern. Once you change the potting mix water only the surface soil is dry. Dig a few inches deeper and check if there is moisture. If there is then there is no need to water.
Normally succulent plants require a loose sandy potting mix. Does your potting mix have enough sand?
The ideal succulent potting mixture is 45% sand, 30% garden soil, 20% compost, and 5% perlite. Alternatively, you can use activated charcoal. Perlite and activated charcoal can absorb moisture. Adding a few pebbles to the potting mix is also beneficial. (potting mix source)
I think it better to replant it in a new potting mix.