Ron, I'll bet it's from the rain. Mine do that too, but only after a few rainy days.

Hey there, I have been doing some research on what diseases can show up on lotus and I am sorry to say there are several that can present problems. The most common are Black Spot and Brown Spot which are unattractive but not really harmful. More of a problem are Blight, Anthracnose, Fusarium wilt and rust. Botrytis, grey mold is common on seeds and seedlings.
Some of these diseases are opportunistic and attack when plant vigor is lacking. Sometimes this is genetic and sometimes it is due to poor culture of seeds or seedlings. Weak plants should be destroyed to avoid continuation of poor plant stock.
Seeds should be soaked in Hydrogen peroxide and water at a ratio of 1:10. Fresh solution should be added every day until the seed has swollen and is ready to split it's skin and extend stems and roots. This is a time for extreme patience. Scarification or cutting the shell of the seed hurries things along. Just don't cut too deep. It can still take 20 days.
The Anthracnose and Fusarium wilt are the most troublesome. The fungi that cause these are not treatable and the plants should be destroyed. The soil should not be used for 3 years or should be sterilized by solarization or chemical sterilization. Any plant debris should be bagged and not used in compost.
I could not find good photos of the various disease process in lotus, only in beans and tomatoes. The lotus is related to the beans, so the photos of the fungus should be similar. The Fusarium will result in root rot after killing the leaves. The Anthacnose shows up as spots that can overlap and penetrate the leaf.
Blight will kill the plant quickly. Leave turn yellow and curl and crinkle, then the root or tuber dies. Blight is a bacterium.
I'm sorry I can't correctly identify which problem your plants have. What I have above is really skimming the surface. Your best bet is to use a fungicide after removing the affected leaves. A fertilizer that contains micronutrients (iron in particular) should help, too. In the future pay close attention to the soil and don't let the seeds be exposed to cool temperatures once you have started soaking them.
The leaves with the small irregular spots are possibly Brown Spot or it could be an insect. The black edges are probably a fungus. Sandy

Are all the leaves affected?
I don't know anything about lotus diseases, since I haven't encountered any problems yet (I'm sure I will eventually!), but I did want to mention that one time when I fertilized a water lily plant, the leaves in one area turned brown. I think I ran into a root/underground stem, etc. Is it possible that when you put your fertilizer tabs in, you damaged something?
What fertilizer do you use, and how often do you feed them?

I just looked at the pictures again and it appears there are circular spots in the dead edges. According to a description of Anthractnose those might be spore bodies. If so, the disease will spread. remove those leaves and bag them for disposal. Sandy
Ps I would give you a source but I'm late for an appointment now.

Thanks for the video link; it was really helpful for my situation, too!