Brown/rust spotting on seedling leaves top-fed DWC

Hey guys, this is my first post for my first grow. Ran into an issue and decided I need some advice or at least to talk through my thoughts. So it is day 13 from seed, growing DP orange bud in a top-fed DWC system. On day 7 they had roots down in the water so I added some nutes at about 1/4 label strength for seedlings - using the GH trio. Beginning PPM was 100. Had some earlier issues with droopy leaves but mostly fixed it by increasing the too low RH and dropping the pH which was in the mid to high 6's.

Now, the plants are starting to show little rust-colored brown spots with darker green coloration around the brown. I was using some sort of heavily filtered water (just like a 5 gal jug of water from home depot) and didn't think to supplement the baseline nutrient levels before mixing up my solution. PPM right now is 100-110, has stayed pretty steady since first adding the nutes last week.

The plants have yet to fully kill off the cotyledons so I'm not sure how much nutrient they're really taking up but read from a few sources 100 ppm was a good choice at this stage. Also- since the PPM and pH have stayed relatively stable at 100-110 and 5.8-6.0 respectively since adding last week and they're drinking water wouldn't that suggest they're at a decent concentration for their current needs? Does GH formulate their trio with a certain assumption of starting cal/mag levels? I have yet to up the ppm to closer to label strength because I'm being a bit overly cautious with nute burn prevention until I'm more familiar with the set-up in action and have heartier plants, but this may also be the source of my issues.

Other info- res temps almost always at 67 f. Might be a little cold air temp-wise but I figured the cfl's I'm using as a light would keep the immediate area warm enough since they're only a few inches from the plants. RH at 65%, lighting with a rigged up 9x23W 6500K cfl in a similarly rigged up reflector.

I will get some pics up shortly (I know it's hard to offer advice without 'em but just wanted to get this post up while I was fretting). Sorry for the lengthy first post - I'm a bit loquacious and currently in concerned parent mode lol. Any help or advice you can offer would be much appreciated! Apologies if this is on the wrong section or something, like I said totally new to forum usage other than quick google searches.

Here are some pics... I'm pretty convinced it's a problem from using RO water but I have calimagic and plan to refill the res with ro water and adding that before the nutes and see how it goes. Also, last pic is of a random free seed I was growing in case one of the other 3 kicked it but I sacrificed it for light repositioning, not gonna have enough room for 4 plants anyway. The rockwool cube has some weird brown color on the side of it, what could that be? Any advice would be awesome

If anyone could give me better advice than I can glean from checking out pics of symptoms it would be great, developed a few more brown spots overnight and I'm getting concerned...please help!

Not sure just how much that might help, it can be hard to read correctly and identify the problem without previous knowledge or grow experience.

I have had this same issue throughout my whole grow (first grow) to some extent. I've only noticed this issue really on 1 plant though and it's roots just happen to lay in the water where I add everything into my res. I thought it was a pH issue, then maybe nute burn? Still not 100 about it.

In dwc you need to start nuets from the beginning because the water has nothing in it. did you soak your rockwool cubes? If not you should of with a weak nute solution phed to 5.5 like 350 ppm. They don't look to bad though bump up the nutes to 350 ppm but add 100 ppm of cal mag first (450 together) then give it a few days and see what happens. And 67 is a good rez temp you want to keep it cool to prevent root rot

I say up your nutes. I did the same thing..put a seedling in my DWC and no nutes for the first week, then i went to 1/4 strength the second week. I was getting the same spots, ended up being underfed, like zchopper420 said DWC needs weak nutes right from the get go. i jumped from 1/4 strength to 3/4 strength the next res change and they jumped right back.