I forgot to mention, I live in Asia and I don't know where to get pine bark fines; that's why I chose to use turface, akadama, perlite and peat as my soiless mix. However, this mix holds on to the water too well, I have them in 8cm cups and they don't seem to dry off for many days. Therefore making watering more often with weaker solution not possible. I would love to be able to water everyday or every other day. Can I take out the peat completely? Is the purpose of peat just to add water retention?

milocrab, you have got me all over the globe - Hong Kong, Asia, Australia!
About your seedlings; the growing medium appears too heavy for a tiny mustard seed and you may have compounded the problem by your watering program. A seed needs to be in a moist environment to imbibe moisture in its own sweet time. If the medium is too wet the seed will rot.
As the seed germinates it uses up its stored energy to produce a root and shoot. These are necessarily small, definitely immature and very sensitive to the environment. Application of fertilizer and exposure to (sun)light must be done carefully and gradually or the seedling can easily die.
All of the leaves in the photographs show a nutritional problem; either a shortage of elements or else a problem with uptake if the elements are present. Overwatering can prevent uptake by displacing oxygen at root level
The bleached areas on some leaves can be caused by sudden and perhaps extreme exposure to (sun)light; or damage due to the activity of the "blotch" (as opposed to the "serpentine") leafminer.
91ðF "even for one hour" to us can mean "for an entire hour?" to a seedling as it pulls on all of its meagre reserves to cope with this environmental stress.
Allow me to reproduce from memory, a poem by Kate Brown from my kindergarten school days.
"In the heart of a seed, buried deep, so deep;
A dear little plant lay fast asleep.
"Wake!" said the sunshine,
"and creep to the light".
"Wake!" said the voice
of the raindrops bright.
The little plant heard it
And rose to see.
What the wonderful
Outside world might be."
We had to mime as we recited and it is a pleasamt memory.

Hi Ronalawn82, thanks for your reply. I used to live in Australia, but now moved to Hong Kong. I had problem registering with my HK address because we don't have postal codes, so I used my old Aussie address instead. :-p

I think I'll have to make another batch of soiless mix to make it less water retentive, and start over again. Perhaps just half perlite and half akadama and leave out the peat and turface altogether. I'll also replace the lime with gypsum and Epsom salt, since there is no peat, the PH of mixture should be around 6-7.

Another thing I can think off is the fan, might be too windy for the seedlings.

I had a similar experience with vegetable seedlings a few years ago in a good quality packaged mix of milled bark, peat and perlite. I believe my problem was actually over fertilization. You pictures don't look like classic fertilizer burn, but something is drawing water out of the seedling which suggests an imbalance. If I were you I would leave the lime, gypsum and epsom salts out since Foliage Pro supplies calcium and magnesium. I would water with distilled water and leave out the vinegar as well. These days I am using a mix of peat and Turface for germinating seeds. I water every two or three days and only add the tiniest bit of FP to every other watering. I never have problems with soggy soil or fungus using this method.

I think you're over-thinking things. I agree with Robin, in that I would suggest you leave out the lime, gypsum, Epsom salts & stick with regular applications of the FP fertilizer.

There's little sense in using all those wonderfully gritty ingredients if you're going to clog thee pores with peat, so I'd skip that. You may also want to consider adding back in a fraction of screened Turface - depending on what you find your water retention needs to be.

I looked at this thread a few days ago - the first impression I got was that there was a high level of TDS & heat in play. I didn't take it as a light intensity issue, but I thought that it might be related to ambient temperatures + the relative heat gain from the photo load. Combining those with a high TDS count, or even those conditioned w/o the high TDS is enough to cause the symptoms you have.

Using only the same ingredients you mentioned, I would strive to eliminate the fine particles or at least wick the pots (to help eliminate excess inter-particular water) and structure the soil so you have enough intra-particular water retention.

Al

Thank you so much for the suggestions! I'll definately try again without the peat and lime (or gypsum). With the temperature being too hot (~75F, measured at the top of the container), there's nothing much I can do to lower it further since the T5HO is hot and I need to place them close to increase the light level.