Fungus spread on seedlings

So a week ago I noticed these spots
Thought it was a calmag deficiency and all I did was water with cal mag and ro water ph 6.0
So I did way more research as I noticed it was spreading to surrounding plants and leafs
So I believe it to be a fungus
It has yellow spots with small black heads that look like fungus
I havnt seen any bugs at all
What do I do to prevent spread?
Humidity is at 45 but I’m buying a dehumidifier now.
is there a product I can use? I plan on transferring to ocean because the stuff I planted my seeds in wasn’t very good soil (mg seed starter)
I’ve already removed the leafs infected from my babies
How bad of a hurt will this put on seedlings with 3-4 nodes?
I think the small white dots are bugs; I don't know what the bigger ones are, but no look like fungus to me? I wouldn't remove any more leaves
Probably too late here but hydrogen peroxide at 3% dilution or less...spray them mist them best advice i can give
If anyone wants to know
I am extremely great full for the information I can revert back to these suggestions heaven forbid something like this would happen but this was a nutrient deficiency. I never fed my plants because I wasn’t sure when to start my first feed so I didn’t and I knew nutes hurt a seedling. I talked to my local hydro store owner and told me to blast them with nutes. I did and since I havnt had a single problem.