Is it indoors? How humid is it there? What kind of 'soil' is it potted in? How do you water - how freequent and how much? How do you determine it is time to water? Looks like fungus and oedema, but just guessing.

They are both outdoors on a plant stand. We are in New South Wales, Australia and it has been between 25°-30°C this month and between 50-80% humidity. We have them both in Scotts Osmocote Professional Cacti & Succulent Mix. We had a pretty decent downpour about 3 weeks ago which made me think it could have been oedema but we got the Cotyledon after the storm and it still got all the black spots. Other than that we only water about every 10 days or so (unless it rained during that time, then we postpone it a couple days).

Did you spray anything on the plants before the spots appeared, any kind of pest control or other chemicals in the air?

Yes, a couple of our other plants have had holes in them from some kind of pest so I sprayed those ones with Yates Pyrethrum Insecticide

m,

I'd guess your mix isn't porous enough. Many commercial mixes, while calling themselves cactus and succulent soils, really are suitable out of the bad and unadulterated for jungle succulents only. Your mix may contain too much bark or other organic matter. I'd cut it with 50% perlite or pumice or small gravel. Those diseased leaves will eventually be replaced by new growth, which are hoped to be fungus-free, but I think your present mix is at the root of your problem here.

If the black spots were not on the plants you sprayed, then you sprayed and the black spots appeared on the treated plants, I think you might of had a bad reaction to the pyrethrum :/

I do not know what bad reaction to some sprays looks like, but it is possible. Are the spots only on plants that were sprayed? - that would help to indentify the origin. There are just too many spots. If it is reaction, leaves may dry up and die, and be replaced by new growth. Or they may stay, just look bad until replaced. Just like in humans, it is best not to overreact and spray plants just to kill possible bugs. It is very important to know what causes damage, if any, and use appropriate treatment. Many natural predators would be killed by some sprays, and they probably could have get rid of few bugs. I am not trying to preach, but a warning.

I agree with cactusmcharris about soil: succulents should be potted in fast draying potting mix. Adding sifted perlite is simplest way to improve drainage. Decreasing organic mater helps - many bugs feed on decaying organic matter. It also retains water, often for too long. And if wet, it could harbor fungus too. Making sure containers have drainage hole (can't tell if yours have or not). Good air circulation helps a lot, especially in hot and humid climates. That is sometimes more difficult to maintain.