plants have become long and spindly

[Choi]

I have only recently started to experiment with hydro and have a small setup at home. I tried Pak Choi in an NFT system. I bought the plants as seedlings and washed the roots, inserted them into tubes with a small block of grow wool for support. Instead of the leaves forming a spinach type bunch as I believe Pak Choi and Bok Choy etc should the plants have become long and spindly with the leaves separated 30-40mm along the stem. The plants have been in for around 6 weeks and are now starting to flower. Is this normal for Pak Choi in hydro? Has anyone had similar with chinese cabbages?