I am new to hydroponic growing and I followed a few tutorials on the basics and in doing so I guess I missed a step when I sowed the seed because, like I do in soil growing, I sowed multiple seeds in to the rockwool. I have a bit of an issue with some seeds that sprouted that look like they're suffering a bit. I was thinking of just clipping the weakest ones and hope that everything except the strongest one will die. My spinach and onion look like they are suffering the most here.
Culling the weak is an important step in multi-seed plantings. (Or "Selecting the strongest" as I've heard it called in recent years.) Eliminate under-performers as soon as they display their under-performance, and don't wait too long to cull the weaker plants from all of your cubes.
Also, I recommend going down to 1 seed per cube. Rockwool says 1-2 seeds, and seed packages may say more, but if you're decent with the process you'll experience a germination rate >95%, and culling the weak is just extra, unnecessary work.
This seems like a decent enough tutorial video (he talks about seeds at ~3:50): https://www.youtube.com/watch?v=T7FZ45hEsEA