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.

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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

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