I started some seeds about 3 weeks ago, some of them looked fine but a few of them looked pretty ugly. Please help me diagnose what I'm doing wrong. Here are some pictures:

This is a mustard green seedling, the cotyledon have beige colored spots on it.

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The first true leaves are getting soft, developed light-colored patches; some of leaves eventually dried up.

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This is one of the collard seedling, they looked absolutely fine before, but now a few of the true leaves start to curl up at the edges, becomes soft then dried up and die.

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This is my first time planting something; there are a few things that I know I did wrong at the start. Firstly, I water everyday, now I know I only need to water when the top layer of the soiless mix is dried. Then I had the seedlings under two T5HO 39W tube; I didn't measure the temperature until a few days ago, and it was up to 89F. I added a fan now and the temperature is lowered to 75-78F. Do you think its the heat + overwatering that is causing the problems now? I have snipped all the leaves that looked bad, do you think these seedlings can still be saved? Thanks.

Bonnie