Rusty leaves accompanied with other signs.

Phosphorous deficiency caused by needless flushing multiple times..........nutrientt salts takes weeks to build up and is impossible to get a salt build up in 2-3 week old seedlings with "normal" fertilizing, even in pre amended soils. Tap water will never have enough cal/mag on its own.

Hard water will not have enough Ca nd Mg on its own. This is not a safe assumption. I have 300ppm hard water. I still need 100+ ppm of Ca and 75ish ppm of Mg in my formula to avoid symptoms of deficiency. I've tracked elemental ppm since i started growing 6+ years ago. I've used hard water and i've used softened water in that time. You really need to include both Ca and Mg in your feed if the soil does not provide enough. HArd water will only account for a few ppm as it is not all calcium and not all of the calcium is plant-available. Devil's advocate -- do you allow the substrate to dry at all between irrigations? This could also be from watering too frequently, as the damage is really tight on the veins. Otherwise, definitely looks like a Ca deficiency. Just dot your I's and cross your T's to make sure it's not being inhibited by something esle at too high of concentration or pH being off.. Ca quickly is locked out if too acidic. It'll be the first thing locked out with a low pH every time.. chemistry is consistent like that.

Iv had a look through your diary, my thoughts are similar to the other growers, they look over watered, could be from the mini flush after the flush. You need to get a proper watering habit. Water/feed until you get runoff, nice and slowly water, takes me hours to water mine. Then wait for it to dry out quite a bit before next watering/feed. Eg my current plants are 10 weeks old and are taking 3 to 4 days before they need watering.

The rust-colored spots are most likely due to a nutrient imbalance from salt buildup in the soil – not a classic CalMag deficiency. Even if your water has enough Ca and Mg, high EC or uneven nutrient levels in active soil mixes can block uptake. Let the pots dry out well before the next watering – overly wet soil can make things worse. Then use only plain pH 6.2–6.5 water for a few cycles. Don’t feed anything new until fresh growth looks clean again. You're likely just dealing with a temporary imbalance.

Your soil is still wet. It will take time to recover. You have to let the pot get really dry. When you look at the soil and it starts to look dry and the whole pot is extremely light when you pick it up. I assure you plants like drought way more than soaking wet soil Like my plants I watered yesterday 5 of them it has been 10days sense I watered them and the other 3 it had been 7days and my plants ae slightly larger than yours and I’m sure my roots are much more established You have root damage at this point. When you let the soil get really dry the plant starts to grow new roots After 2 or 3 water cycles of waiting till soil is dry as a bone and then water you will have the roots continue to grow Everytime you water to soon all the new roots get all wet and then brown and start to die off I understand it can be deceiving thinking it time to water b4 that cause you will see the plant perk up at times of the day but then it won’t do it at other times you cants watch the leaves to know when to water unless your plant is healthy A stunted plant acts way different Lowering light intensity for a week will help Plant will not start to look healthy till you water correctly several times in a row so just don’t even look at what the plant looks like for the next 10 to 14 days only look at how dry the dirt is and fell how heavy the pot is you want the pot so light that you think the plant will die off because there is absolutely no water in the dirt