Hard, crusty, sticky leaves. Yet soil is dry and seperating from pot sides!? Help pls

I would personally just hold off on the drip system and do it by hand. Water til you see a little runoff then wait til its mostly dry and repeat. It could be around 3 days before youd have to water again depending on your setup. Yellowing usually will mean that the plant needs nitrogen. Not always. This could be from not enough nutes or your waters ph is too high or low for the plant to uptake the nutes.
Other than watering, the first thing i would recommend to do is get yourself something to test the ph of your tap water. That digital pen i recommended a couple posts ago works great.

Ok im trying to get hold of a digital ph pen but looks like i may have to get it online.. in the meantime i grabbed another ph tester that has the metal rod and the dial.
The device i got is meant to be for soil however but i did test the ph of the tap water. After a minute or so (the time it says to leave ot in soil) the ph was showin 6.2 however if left longer it would eventually go down to 5.2 at which point it would stop. im guessing this is the true ph of the tap water?
I then checked the ph of the soil and the ph of the water in the tank (with nutes) and both times the dial remained on ph 7.

In the last few days ive done what you said and watered by hand untill i see a little run off and then let it dry out. ive found myself waterin twice a day.
The plants seem to still be growing fine and the new leaves look healthy but the bottom bigger leaves on the main 'bad' plant look in a bad way.

Ill post some recent pics....

Really appreciate the help

Attachment 280451

The metal probe ph testers are junk. They are not very accurate at all. You might get a more accurate reading with the aquarium ph test strips (Walmart should have them) until you can get a ph meter.

Ok, now what i meant by letting it dry out. When you water slowly til runoff the pot should be heavy. Take a mental note on weight. When that pot gets light then its time to water again. 18L is almost 5gal i think so it should take at least 3 or 4 days to dry out. Do not water it anymore. Twice a day is way too much. If its drying out that fast you have other problems.

For future reference IMO its better to transplant into different size pots. Dont put little seedlings in a big pot.

Im thinking once you get your ph straight and get a watering schedule down that plant will be doing alot better. Just remember they like a wet dry cycle. Then you can start adding a small amount of nutes once everything is on the right track. If it gets any worse post this in the plant problem section and someone will speak up if im missing something.

Would you suggest transfering them even now at 4 weeks old and about 12" high?

Something else that is confusing me abit is that the three other plants under the same conditions and watering schedule seem to be pretty healthy... One has slightly yellowing leaves near the bottom however that seems to be getting better since i changed the watering schedule. Is it possible that i could of over sprayed that one plant or something like that has made the leaves go hard? Also the new leaves coming through on the bad plant seem ok...?