Couple snags in my first DWC attempt....

This is my first real go at hydro, and I've ran into a couple little snags. All the plants are looking healthy except for these 3 little ones. One of them has it's first real leaves, but the second set are not developing. I've been looking around the internt for an hour now, and it's driving me nuts what is causig it. Is it just a mutant? The second problem is the discoloration of this plant. It has small yelowish patches on the leaves that it has. Almost looked like light bleaching at first, but I'm not really that sure. The third picture is a plant that is growing it's second set of leaves, but they're starting to curl downwards. Too much nutes? All my plants are in the same reservoir. pH has been at a consistent 5.5-5.8, and ppm is at 350 right now. My straight tap water is almost 200 so that's why it seems a bit high. Any tips would be greatly appreciated.....

 

I actually got some calibrating solutions for my pH and tds meters the other day, and discovered that both were off. My pH (which I thought was at 5.7) was really running around 3, and my tds was running to high as well. I've since fixed the problem, and all the plants have gotten healthier, and had much new growth!