Silica hardens stems and helps maintain a strong plant. However, it does take awhile to work, so if you use it, use it early on.
Purpling stems could be from a p- but before you go dosing it, Id def take shroomkings advice. Water less often, and use less water then you have been when you do. If Water gathers and roots are allowed to sit in it youll get root rot, which can cause all sorts of problems. Allow your soil to mildly dry out before you water.
A good rule of thumb, that i use, is to dig into a little bit of top soil. If its dry when you dig down as deep as your fingernail or first knuckle, might be rdy to water. Remember when soil dries a tad it causes roots to stretch hard and attempt to find some water. The roots also need air. More roots = more vigorous growth. Keep this stuff in mind, IMO. Good luck man, have fun with it and dont forget to read read read!!
Peace
Gguy